When writing SF/F, you get one impossible thing for free. Everything else you have to earn.
Put another way, you can't make everything up. Generally speaking, stories have to have enough grounding in the naturalistic world for the reader to relate to them. (There are of course always brilliant exceptions to this and every other rule of writing, but they're damned tough to pull off.) Likewise, if you're going to ask the reader to swallow something huge and improbable, a bunch of sweet reason can help it go down.
It's clear enough this rule isn't literally true. Plenty of science fiction comes with FTL travel, strong AI and teleporters all at the same time, for example. But in a sense, those are all one thing. Say, the starship Enterprise.
But if you want the full starship package and vampires for the crew, you'd better make me believe in what you're doing. Because I can buy the starship thing. That's one of our tropes, what Gardner Dozois calls "the furniture of science fiction". And I can buy the vampire thing if you're writing urban fantasy or horror.
But vampires in space is a real (if interesting) stretch. I mean, what about that whole sunlight thing? (And for that matter, what happens to werewolves who go on a lunar expedition?) Vampires on a starship... Now you've added too many impossible things. Unless of course you've earned it within the story through world building or character or plot.
The other end of this phenomenon is what John Scalzi calls "The Flying Snowman", where the impossibilities are all being accepted until the suspension of disbelief is shattered by something that goes too far over the top. I believe this is just the same principle written from the opposite direction.
So, yeah. You get one impossible thing for free. That comes on credit from me, the reader. Everything after that had better make sense, at least within the internal consistency of the story being told.