The flute mistakes don't bother Jiang Cheng or ruin his concentration, though he does pick up on them.  It's one of the rare benefits of being someone who is merely passable at music compared to someone with perfect pitch.  He wouldn't want someone who made so many mistakes performing for an audience while representing him, but Wei Wuxian is neither performing nor a representation of the Jiang Sect's musical teachings.

Jiang Cheng debates for a moment whether to reuse the tea cups, get out clean bowls, or just drink from the jars.  He had bought two different varieties with the intent to let Wei Wuxian try both, so not the latter, even if the fact that he'd automatically passed one of the jars over in getting Wei Wuxian's attention has already complicated that plan.  It's no great loss - they aren't wildly different varieties - but keeping the option open might be nice if he can arrange it without admitting he made a mistake.  As for the other options, he's already put the cups away and wouldn't be able to tell which was which, so he's left with the clean bowl option.  What are two more dishes?

Thinking takes enough time that he's still rummaging in his bag for bowls when Wei Wuxian sits down and starts watching him.  He slides one over, then pours wine from the jar still near him into his own bowl before setting the jar closer to the middle of the table.  

"My Wei Wuxian would have just grabbed a jar and immediately started drinking from it without waiting for me to find bowls.  Most likely spilling half of the wine over himself in the process.  So, good job on being more civilized than your counterpart," he says, because Jiang Cheng does not miss his Wei Wuxian and has no wish to ever see him do any of his familiar habits.  And because they aren't standing on any ceremony, he just drinks from the bowl without bothering to come up with a proper toast beyond that.  

If Jiang Cheng is looking at Wei Wuxian at all, he will probably notice that Wei Wuxian looks extremely surprised at the pouring wine. Not unpleasantly surprised - in fact, quite excitedly surprised!

(He gets alcohol?! - Apparently he was totally correct about jar familiarity actually! ...Is this - he remembers that people in the past used to be freer about alcohol, because water purification. No, they already had tea and other Jiang Cheng can boil water with a gesture, that makes no sense as an explanation here. Cultural carryover from the people who can't boil water with a gesture...??)

He follows other Jiang Cheng in pouring, which promptly and decisively eliminates the possibility that really it was only other Jiang Cheng getting alcohol, and he was getting some other thing. (He carefully pours no more than other Jiang Cheng did, erring on the side of less if there's no convenient mark or anything.) Sets the jar in an almost-mirror of other Jiang Cheng's. 

...Whaaat is he supposed to say to that. (He winces (not in the flinching way) just a little. He can see the appeal! But wow that can not have been a good resultant time for other him). (This is not making him less surprised that other Jiang Cheng is serving wine to any Wei Wuxians!) "I'll try not to spill, Sandu Shengshou." ...Well that was not literally the worst words that could have fallen out of his mouth! Brain, Wei Ying, use the one in your head. "Sorry!

- It's generous of you to be so patient with us, Sect Leader Jiang." ...Either that was better or he's not going to get better and should instead shut up.

...But, hey, other Jiang Cheng is happy enough with him to get him wine, and in related positive facts, he has wine! Cool alternate universe/historical wine! Other Jiang Cheng does not seem to have said something that would plausibly be a toast. He raises his bowl cheerfully. "To your health and the prosperity of the Jiang Sect, Sandu Shengshou!" (...Possibly he should have stuck with the shutting up until told otherwise - does he know anything at all about alcohol practices and toast etiquette here, no he doesn't. But, too late now, and other Jiang Cheng's been so nice with teaching him, and come on he should get some 'alternate universe visitor' credit?)

Wine! Fortunately other Jiang Cheng has already started drinking so he doesn't have to make himself wait. (Music camp had some strict rules about alcohol, and didn't have much affordances for sneaking any either even if he wanted to risk it that badly.) Glug glug (careful not to spill) wine!!

Jiang Cheng doesn't bother justifying his purchasing decisions with the local history and customs of alcohol.

He closes his eyes for a moment to focus on the taste.  Its the same as it's always tasted.  Jiang Cheng tries to imagine drinking it for the first time, trying to ignore how self conscious it makes him to have the local varieties compared to whatever the other world's wine is like.  This Wei Wuxian wouldn't have the nostalgia for it that his does.  Even with such familiarity, his Wei Wuxian was constantly going off and praising other region's alcohols.  

He drinks his own bowl in silence, unable to decide on anything worth saying.  When they've finished, he nudges his original jar a finger's width closer to Wei Wuxian and grabs the other.  "Try both.  They're different." 

Once he's poured another bowl, the jar is put back in the center of the table.  Presumably Wei Wuxian will take whichever one he likes more, or alternate, or some ridiculous third thing that Jiang Cheng will wind up yelling about.  Until then, he sips at the second bowl slowly, hoping Wei Wuxian will fill the silence.

(Wei Wuxian was not expecting him to!)

He - does not copy other Jiang Cheng in closing his eyes; other Jiang Cheng isn't going to be able to tell if he does or not (...probably. Unless he has some power use about it) because: eyes closed, and if he closes his eyes then he's not going to be able to watch other Jiang Cheng to know when he might be supposed to open them again. Also he will be way more likely to in fact spill the wine, which he just said he was going to try to not do.

He gets more than one alcohol?! "You're so nice, Sect Leader Jiang. -Yes Sandu Shengshou, thank you Sandu Shengshou!" Enthusiastic (careful) pouring! Different wine!

Expecting this Wei Wuxian to fill silence without telling him he should be doing that would generally be a much less reliable hope than with the local Wei Wuxian. (It's not lack of interest! It's all the not-speaking-at-doms-when-not-spoken-and/or-permitted-to education.) But at the moment this Wei Wuxian was already subspacing, and has now also drunk a bowl and some of wine (what with not being and never having been able to just walk into a bar or other establishment and obtain alcohol whenever he might want it, this Wei Wuxian has not had the chance to develop quite the alcohol tolerance of his local counterpart). 

And commenting on the food was fine, and offering someone an assorti usually means you want their reactions on the variations, right? And other Jiang Cheng's the one who's giving him alcohol, so he knows Wei Wuxian is here having alcohol, and if he definitely wanted quiet alcohol times he could have said that. (You know who would want quiet alcohol times? Lans. Except they won't because they don't have alcohol at all.)

Oh, yes, he can taste the difference! They're both very good, thank you Sect Leader Jiang! He can elaborate on some qualities they have! Are they from - sorry! If he can ask, are they from different places here, or something else? Do- if he can ask! Do they have a lot of kinds of wine here? In his world they have a lot. Some of them are made with advances in modern technology, and some of them are trying to avoid that on purpose. He can compare these ones to some he's tried before! If he can ask, does Sandu Shengshou like one of them better? What about other him? If he can ask, and if Sect Leader Jiang knows.

Wei Wuxian notices the jar placement. ...Jar in the center of the table has been having a communicative meaning here. Does that mean he's allowed to take more? ...What if he kind of slowly starts reaching towards the jar, is there going to be a "Wei Wuxian!" (Or a 'what do you think you're doing' -  which is the same thing, really.)

Jiang Cheng continues to be weak to complements.  He attempts to huff and says "Well, it's about time someone acknowledged that" in a way that utterly fails to hide how flustered and pleased he is.  Jiang Cheng is going to blame that on being drunk.

It's interesting to hear how other places describe alcohol, and to have theirs compared.  (At least, he's thinking that since the feedback seems to be mostly positive.  Lotus Pier itself doesn't manufacture wine, beyond a few hobbyists making it for themselves.  Still, it's hard to not be defensive about the closer villages too when speaking to someone from further away.)

"Both of the wines are from Yunmeng, though not the same brewer.  I like the one with the green stripe around the jar better," Jiang Cheng shrugs.  Then, knowing what Wei Wuxians are like, has to add, "Don't save it specifically for me.  I can go out and buy more whenever I want.  I don't know if my Wei Wuxian has a preference since he drinks so quickly I doubt he even tastes them."  

"A great many things can be turned into alcohol and people have been thorough in trying.  Rice, fruit... Wei Wuxian created a variety with lotus seeds and leaves once.  It tasted okay, but I much prefer the use Wen Ning put it to - poisoning it and feeding it to his awful family.  ...Ah.  Wen Ning is one of Wei Wuxian's friends.  The one who died and is now following Wei Wuxian around as a fierce corpse."  Jiang Cheng recalls complaining about him, back when he thought this Wei Wuxian was his Wei Wuxian.  Someone being brought back as a fierce corpse seems like the sort of thing that someone would remember hearing about, but there's no knowing what Wei Wuxian's memory is going to latch onto or abandon.