‎ ‎ Why Do Vampires Sparkle In The Sun?

It is my understanding that vampires disintegrate upon contact with sunlight. Why do vampires from Twilight sparkle?

Vampire flesh is harder than human flesh and their skin is many times smoother. Part of this is due to their venom. It replaces all of their bodily fluids, including sweat. They still exude it, however, but at a much lower rate than they used to. Upon exposure to bright lights, their venom crystallizes and starts to catch the light, refracting and reflecting it. This also helps their skin feel smoother, as all of their pores are being filled with crystallized vampire venom.

So why does the venom melt again when the light supply is cut off?

It doesn't melt, it powders over time and floats off. Which is why homes where vampires live still get dust, it's the powdered venom crystals.

Immediately after sunlight stimulus is removed?

It takes a little bit of time. Probably a few minutes.

The type of "vampire" noted in Twilight is not true "undead" (just like the wolves of the Quiluete tribe are not true werewolves but more akin to skinwalkers). They have reflections and silver and crosses do not affect them. They even have a pulse and working organs (including sexual organs to an extent). Their flesh is transformed upon becoming a "vampire" so that it is rock hard, often described as "marble". Due to the millions of tiny variations in human skin, when marbelized and exposed to direct sunlight it causes a sparkling effect due to the many angles created by the nature of "living marble".