What's a good scientficic reason for vampires burning in sunlight?

They do not. The myth as stated in Dracula (which is different from Carmilla and earlier works) states that part of the curse of the vampire is that it has to return to its coffin before daybreak otherwise they will die and become normal corpses. If they have been turned for some time maybe even decompose. But, and here it comes the big exception, this does not seem to apply to older, stronger vampires. They do not seem to die when exposed to sun light although they try to avoid it. The named reason seems to be that their powers diminish greatly when exposed to daylight thus becoming more vulnerable. Both the sunlight burning and the mirror thing comes from movies. But in mythology mirrors do tend to show the true nature of things thus being something a vampire would want to avoid since their main power is deceit and seduction.