That's because when you leave a vampire in the sun, it will get the 3 hour -25 moodlet "Heating Up". If the 3 hours go by and it is still in the sun, it will faint and get a 2d -50 moodlet, "Too Much Sun". At this rate, it's Thirst need will drop down really fast, and even though I haven't personally tested it yet, I believe they will eventually die of starvation.