I think that the braces would just pop off. Everything in your body changes when you get turned. All of the vampires have straight teeth. I don't think that they mentioned it, but it just makes sense that their beauty would be complemented with a beautiful smile.
People, obviously, didn't use braces when Carlisle was growing up, so he could have had odd looking teeth, but he doesn't. They're never mentioned so we can assume that the teeth are flawless, like all of the other vampiric features.
Because the teeth change during the change (that sounds weird), I think that the power of the change would just tear through braces. If a person's features can completely change, why wouldn't the teeth change too. To change a person's features would take a lot of power in the venom, so I think it's pretty safe to assume it could destroy the braces. (though that would be pretty weird)