Everyone seems to agree that sunlight is bad; but garlic is a rumor vampires started off (it's an antiseptic - necks smeared with garlic make for healthier feeding). Older ones are more powerful. For immortal beings, they seem to die quite easily; still, Dracula returns more often than the octopus in FinalFantasy 6. They don't seem to turn into bats so much these days, unless it's a large flock; conservation of mass?
It's a bit like serial killers. Given that they spend substantial portions of their life knocking people off, they seem awfully inept at getting rid of the protagonists in most of the books or films I've seen. --Angoel
I think the FlockOfBats? thing is actually older than the single bat - but is making a resurgence because with modern CGI it just looks cooler. Current state of myth varies on who you talk to. Sunlight being a 'crumble to dust' style cure seems to exist only for as long as the camera crew don't find it inconvenient. --Vitenka