So I was just thinking about how Twilight vampires works, because I see this question a lot, and while there's not a lot of hard science behind it in the books, this is how I choose to believe they work and how I will portray them when I write them.
Their skin: This one is one a lot of people seems to have trouble with because they "sparkle". In the books, it's just described as a faint glow in the sunlight, so I feel like the movies overdid it a bit, but the reasoning I have come up with, is that their individual skin cells have hardened due to a calcification (for lack of a better word), and have become like a hard outer shell for protection. Think scales, like for an alligator or armadillo or even a dragon. This is why their whole body is not truly a rock and it's just their skin on the outside, which allows them to breathe and move around freely. The hardening of the skin also creates that reflective property.
Their breathing: They do not breathe all of the time, as it is not a function that they need to survive. Their breathing is used for taking in smells as they are a predator and they need this to track down prey. They often have to remind themselves to breathe around people so that they don't appear so still and unhuman like. So please don't expect them to breathe unless they're hunting or trying to appear normal in front of a lot of people. Your character may pick up on this if my character(s) are not careful.
Their venom: Their venom takes the place of blood. It's everywhere in their bodies and possibly running through their old blood vessels, this helps their abilities and it helps repair any damage done to their bodies and is possibly what helps them procreate, too. Think of it this way... the venom is made in such a way to spread to humans and to make more vampires and making a child is a way to do this, hence Renesmee happened. All and all I think this is something present all over their body and not just around their teeth.
Their appearance: I feel like all of the vampires are made to look beautiful. It's a trap to humans to draw them in, for both food or for creating a new vampire. However, it's entirely possible that humans can pick up that there is something not quite right about them which can also make them feel uneasy about them.
Their abilities: Their abilities tends to come from something they were capable of doing in their human life and the venom just helps enhance this trait of theirs.
A side note: If you have a vampire character, and they're not a Twilight vampire, that is completely okay. We can make it work. We can just say that they're a different type of vampire. After all there are different types of dogs, cats, etc., so it would be too far of a stretch to say that Vampires have different origins too, or different stages of evolution and things over the years.
If you guys have any questions, please ask!
