Leareth realizes he didn't actually specify which of them, but he can do both. 

Fëanáro ran away as a child, when his father remarried after his mother's death; the other Leareth doesn't remember his exact age, just that it was around the equivalent of a human ten-year-old. He invented writing as an adolescent. He married a woman called Nerdanel, whose family had some sort of relationship with Aulë, god of building things. He had a number of very talented children, the oldest of whom was called Nelyafinwë Maitimo and got on well with the other Leareth. He created some powerful magical artifacts called the Silmarils, which captured the light of the Trees.

He tested the other Leareth's authenticity as an alien from another world by immediately asking to learn eleven languages in a day. 

Vanyel's day to day life was mostly unclear to Leareth until 798, when Queen Elspeth died, her grandson Randale took the throne, and the Karsites immediately started a war. Vanyel was the single biggest player in Valdemar's military and became an incredibly famous war hero, though the other Leareth thought he spent most of the years-long war very depressed. He eventually used blood-magic at the final battle to take Karse's capital city and give it back to the surviving princess of the royal family who had escaped a coup. Said princess was possessed by Vkandis and given miraculous healing after Valdemar's victory. (No gryphons showed up, though.) There was an enormous scandal over the blood-magic. Eventually, in 806, Vanyel and Leareth had established enough trust for the other Leareth to share his god-plan. Vanyel immediately blocked the Foresight dream, and (with one dream worth of explanation) went off on some sort of fact-finding research mission to learn more about Leareth's past lives. 

"Aww, he married Nerdanel? Awwww...

- he named him Nelyafinwë?

- anyway he won't speak Pax so I can establish myself by teaching him Pax."

"That sounds good. Why is Nelyafinwë surprising as a name? I think the other Leareth had not paid much attention to it." 

"Nelya means 'three' and in the context of having a fight with his half-siblings is extremely gauche."

"Oh, rudely trying to make a political point via a name-meaning sounds very characteristic of him, at least of what I gathered from the other Leareth. Do you have any other questions?"  

"Do you have an opinion whether I should talk to Fëanáro first or Vanyel?"

"I would honestly expect Vanyel to be more reasonable, and - easier to keep on topic, but that is perhaps my bias because I know Vanyel already." 

"Maybe contacting Fëanáro should wait until my Fëanáro has an earcuff of his own - I'll have to scroll it out for him when I get a chance - and then they can catch each other up on things of relevance to Fëanáros and be fifty percent more focused when talking to everyone else."

"That seems probably wise." 

"Okay. I don't suppose he has any idea what time it is, but I can see if Vanyel's awake from here if I can reach him at all." She attempts to reach - older Vanyel, from the same world as that other Leareth -

–What is that, it's not Mindspeech, it's not osanwë, is it some sort of Maia-power? Vanyel drops the notes he was holding, reinforces his shields just in case it's an attack, and very tentatively reaches back. 

:Who is this: 

We've never met. My name is Bella. I am not from but am currently in what seems to be a separate, earlier Velgarth, and used to live in a separate, earlier Arda. I'm calling to let you know that in a transport mishap earlier today I landed in the Angband of your world and teleported out with the Leareth therefrom; he's currently at my Leareth's place.

:What:

Vanyel's first thought is that this is really, obviously a trap. His second thought is that surely Melkor could have come up with something that was literally any amount more plausible and likely to fool him. 

:What do you want?: he adds. 

...to tell you that. I don't have up to date strategic information on how the war is going because your Leareth's been in Angband for a while so I don't know what to suggest from here, although I guess I can explain what I bring to the mix, or will once my interplanar transit spell is working again? Though it might be faster to wait for my Fëanáro to replicate my comms device and talk to his counterpart.

:Just to be clear, the thing wrong with your, er, interplanar transit spell is that it landed you in Angband by accident? And then it sounds like you got out again fine: 

Right, I didn't want to stick around and debug it in - uh, I thought it was Utumno, but same difference - so I did it again and it got me where I was trying to go, but I want to find out if there's something that's making the spell have a high error rate, such that I could fix it.

:Right. So we should expect contact at some point from, er, a much younger Fëanáro? How young?: 

He might tell you he's almost twelve. That's barely past toddler size for Quendi, though.

:...Oh no that sounds like an unbelievable level of adorable: 

He is!!!! I heard he married Nerdanel, I know tiny Nerdanel too.

:Aww. I haven't met Nerdanel, she stayed behind in Valinor, but she certainly raised some lovely children. Anyway. What sorts of help could you theoretically offer us, if your teleport is working again?: 

Without the interplanar teleport, I can use an artifact I have to close telepathic distance. I have something like a Mindspeaking-Thoughtsensing-Mindhealing mashup power; I can use it from here on targets I know how to specify, which could be useful for relaying things over long distances - I don't think it would be safe to try offensive use against Ainur. It also works for Velgarth and Quendi telepathic abilities for anyone I loan it to. With the interplanar teleport, I can show up in person and use spells - I have a good healing spell, and an intraplanar teleport, and some random utility and minor combat stuff, and I can enchant a few kinds of artifacts.

:Interesting. I can, er, pass all of that on: 

Pause. He has a number of questions, most of which aren't that strategically urgent, one of which he definitely has to ask even though his mind is very flinchy about it. 

:How did you end up on, um, visiting-his-place terms with the Leareth you know over there?: is what comes out. 

Oh, uh, I attracted a lot of attention after I'd been in Haven for a few weeks, I put people's missing limbs back and stuff? And he noticed that and was curious about me and eventually offered to help me if I needed it especially if a god had it in for me. Then Iftel burst open like a horrible pinata and sent a small army after me because the gods didn't like my face and I teleported him in and he helped with that.

:Iftel?: Blankly.