Depends on if i am on his side or not, really. in any case, i wouldn't underestimate the power of the force.

Darth Vader is the shiz.

Dracula would eventually become hungry and kill me, whereas Darth Vader has no inherent reason to kill me (and I would try hard not to give him a non-inherent one). Furthermore, Dracula may have ways of escaping the room that leave me stuck behind (such as turning into gaseous form or a small bat) whereas if Darth Vader busts out he'll probably leave a hole large enough for me to follow him.

With Dracula you get to become a vampire. That's sweet.

I'd choose Darth Vader... at least that way he can tell me if I'm force sensitive or not, then I will begin my training on the Dark Side...

I imagine both are fairly likely to kill you, but I think Dracula would be more likely to. Vader may seem to be a safer choice, but keep in mind his affinity for the dark side; he doesn't adhere to a similar moral set that most of us do. He'd kill you without a second thought if it even slightly benefitted him. On the other hand, Dracula could turn you into a vampire (if you're into that sort of thing), but more likely, he'd just suck your blood and leave you for dead. It's pretty bleak either way, but I'd have to go with Vader since he could use the force to summon help from his Empire or break out. Then hopefully he wouldn't have killed me because I swore to worship him and join his imperial army or whatever he wanted. The only options with Dracula seem to be food or be immortal (I doubt he'd leave you untouched), and immortality just doesn't appeal to me.

Or maybe both Vader and Dracula would be locked in a room with you because they chose a "Be locked in a room with a random mortal and you cannot kill or harm them" WYR option.

@RbrtKurtz - That could be an interesting option