Well, I think camera flash doesn't quite follow the same properties as sunlight. I'm not a very scientific person, so I can't be sure. Sunlight is basically white light, so when it is passed through a prism, it splits the light into seven colors, due to dispersion of light.
The Twilight vampires' skin is made of a substance that carries the properties of marble and crystal. When sunlight falls on their skin, the light gets dispersed, causing the "sparkling effect."
I think the light emitted by a camera flash might not consist of quite so many colors, and it's also possible that they are monochromatic. Fluorescent light might not be composed of seven colors either. Even if they are, the resultant dispersion might not be as effective as sunlight passing through a prism.
All this might be pure rubbish, I don't know, but I hope this makes some sense.
Good question, by the way. :)