The World of Darkness had a pretty reasonable explanation for it.Thedayrecker said:Do you have to drink blood? That's always bothered me. Vampire movies rarely explain why vampires need to drink blood. If I didn't have to, I'd become a friendly superhero, and subsequently starve.
Blood is basically their form of energy, they use blood waking up each night (no blood = you stay asleep/ go into torpor) and the blood is spent to fuel their vampire abilities, each time they use an ability it costs them a certain amount of blood so they constantly have to replenish.
OT: It really depends on what kind of vampire you're talking about, the lore/rules around vampires changes quite a bit in different forms of media. But I'd probably do a lot of stuff I haven't been able to do, dabble in various things I've always wanted to do but have been unable due to financial constraints/ school/ daily life.
I think I'd go about a fairly relaxed existence with the burden of most parts of daily life lifted off me.