My thought on it is simply that while there may be another big cycle after Hell on Earth, that doesn't necessarily mean that the next cycle is set after Hell on Earth. It could be that Hell on Earth really is the end of the B.P.R.D. series as we know it, and the future is in the past with Hellboy and the B.P.R.D..
Or it could be that the next cycle is simply an evolution of what came before it. Plague of Frogs was not an isolated incident that wrapped up nicely before Hell on Earth kicked off. Plague of Frogs was the beginning of the end, and Hell on Earth was the next step. Mike Mignola has made a few very vague references to a post-apocalyptic world in some interviews. Maybe that's where we go next.
The vampire thing can be a part of all this, but I don't see it being the major thing ever. John Arcudi doesn't do vampires. And they don't have the same scale as Ogdru Hem (in a thematic sense).