She takes to being a vampire very well. She noticeably has a temper, but she's usually more angered by the urge to rip something's head off than whatever caused her to want to rip something's head off. This translates to occasional growling and stomping off to brood instead of ripping off limbs or attacking anyone. Her power becomes a good port of call when she's upset: even when she's not trying to figure out the lengths of her power, she seems to find it very soothing to push transmutation at objects.
Blair is very good at dealing with her, and avoids her wrath almost entirely. Edward is not so lucky, but Blair thinks helpful instructions at him and this helps. They manage to keep everyone's limbs where they belong, though lots of trees get some very passive aggressive makeovers. (Yvette conscientiously puts them back when she's done, otherwise a human might notice eventually.)
Books are fetched for bored vampires to read, along with interesting materials for Yvette to copy; newspaper, glass, cotton, things that he can easily get without stealing.
(Blair's highly tempted to steal something made of, say, gold, but he gets the impression that his companions would find this objectionable, even if he put it back after Yvette copied it. So he doesn't.)
She plays with her power most of the time. There's a lot of nuance to it, once she gets to playing with it. At first it's simply pushing copies of other materials she's made at the entire transmutation area, but then she figures out how to push the material at part of it, and then claws her way to two different materials at once. Then, she works at fine-detail manipulation. Stretching out the area that she can work with is hard - it does not want to work over a large area. It wants to be a small amorphous blob. She growls at it, and then starts figuring out how to shape the blob. If she keeps it as thin as she can get it she can stretch it out further into something to get at it for transmutation. This is tricky. Also not very fun.
She'd like to have a larger library of potential materials, but for that she needs to get off the island. And to get off the island, she needs to be safe.
... She wonders what the plan is for getting her to not be a murderer. Waft things smelling of humans in front of her, test her control without any actual person in danger?
The plan is to have her near an actual human, ideally one who can't see her, with her minders by to decapitate her if necessary. Things that smell of humans might be useful preliminaries but will not actually test her control. They can't duplicate the sound of a heartbeat or the warmth of the real scent.
And then - time to test her self control.
(She's incredibly nervous.)
And they find someone to invite on a canoe ride, and go by the island, while Edward and Blair are standing by Yvette. If she lunges, the human will never know.
She does not lunge.
It's unpleasant (she wonders how the hell Blair managed to casually hold conversations with her while she was human, how any of them managed it), but she keeps herself very still and breathes in and out evenly and runs through her 'I will not eat people' litany over and over in her head at lightning speed. She will not eat people, here are her reasons why, she refuses to be something she does not choose to be and she does not choose to be a killer.
And then the human drifts out of her range of scent. They probably just had a pleasant canoe ride.
"I'm not up to interacting with them yet, I think," she pronounces. "But! You did not need to rip my head off!"
"Thank you." Pause. "I do think I'm going to go eat a deer, though."
Inhaling flames is not fun.
It's entirely reasonable that she eat a deer. (Edward stays within range of her, in case she decides to go for a swim abruptly.)
She eats a deer! It does not cross her mind to go for an abrupt swim; she's busy in some mix of happy excitement and trying to better her methods for not killing people.
But (possibly to Edward's surprise) she does not get cocky. She does not want to leave the island just yet. She'll have a few more unknowing humans sent past her before she's even willing to be visible to them. So a few more humans are sent past her, and she doesn't lunge.
And then, tentatively, she thinks she's ready to be put in front of an actual human, and maybe try talking to one.
Well, they don't have any knowledgeable humans handy, so this means interacting with them in normal human contexts, which means going back to Rochester.
Eep.
Well.
That'll work. Tentatively. Because that's where the vampires are that could stop her, and because she's managed self control several times without fail.
(Such eep.)
Edward will stay right by her and react if she lunges at someone. But maybe she'd like to stick to going out at night, so there will be fewer witnesses and they won't have such good line of sight.
She is very, very careful about this, though. There are lots of people! People are very breakable. Some people bleed monthly, and while this wouldn't send her into a frenzy on account of the hormones in the blood, it still makes it a bit uncomfortable. So she's going to mostly stay indoors and keep away from the squishy humans. She is worried for their safety.
(But she joyously copies all of the materials, look at all of these materials she can copy, there are so many, look at them all! Gold and silver and brick and iron and steel and every single material she can walk up and touch.)
The coven has accumulated a reasonable quantity of more or less useless possessions over the years, many of which are made of unusual materials.
Excellent. She will be an inspiration for pokemon: gotta catch 'em all. All of the materials will be hers.
Meanwhile, except for mostly-Edward, sometimes-Carlisle-or-Esme keeping an eye on her, they go about their lives as normal.
"Yvette, you might want to leave," Edward says suddenly, one late night, interrupting himself at the piano.
"Because Carlisle is on his way home with a bleeding woman he's decided to turn."
She pockets her half-transmuted current project and zips away to pack a suitcase so she can be elsewhere for a while and runs over the things another newborn would want to know and what she'll have to deal with and -
It occurs to her that the bleeding woman might have a similar reservation about vampirism that Yvette might, and that Yvette personally has a way to maybe copy and remake the important parts before vampirism makes it impossible.
"Edward, can you tell if she cares?" she asks, freezing mid-pack.