My very first time in BP, I was checking out the clubs, when I noticed a ruckas and got to a sim just in time to see him grab his throat and then go into a pile of dust and then grim came. Since I had just got the game, I did not know what the heck was going on and the ghost was red, then I started reading about it and found out it was thirst. I have also seen vampires with the thirst bubble over their head and pull out plasma fruit and eat it and this was a sim I was not even playing. So they do this on their own, I suppose, if they have it in their inventory.
My vampire sim left work and passed out. She is so in the red, she has that nasty overexposure moodlet, which lasts two days. Poor thing.