The one with the Vampires is an altogether lighter romp than the previous five episodes, driven by some perfectly harmless gobbledegook. (Naturally, giant fish with cloaking devises don't have a reflection in the mirror, except when they do, because, er...)
Where the one with the Angels was a backdrop for some heavy stuff between the Doctor and Amy and some more clues about River Bloody Song, this was a backdrop to develop the relationship between Amy and Rory. Although he appears in episode 1, this is the first time we've really seen him as a character: in terms of what is going to happen over the next two weeks, it's pretty imperative that we like him. Which, I think, during this story, we pretty much do. He's a buffoon, but a nice buffoon, pointlessly trying to repel the not-vampire with a cross; jealous about the Doctor and Amy but not getting into a Mickey type strop about it. A running thread through Old New Who was that the Doctor is an inspirational figure, changing the lives of everyone he comes across. So it is very pointed and clever for Rory, the gormless normal guy to see through this or at any rate spot the downside of it. The Doctor, just by being the Doctor, inspires people to take risks which may harm them or get them killed.
But mostly, this story was a very agreeable piece of padding; showing us the Doctor and Amy having an adventure and quite enjoying it -- putting across to us the idea that they travel together and have fun travelling together, and that while dangerous stuff happens all the time every single minute isn't as incredibly heavy and angst ridden as last week's story was and next week's story is going to be.
There are Vampire myths in every civilisation in the universe, because the Time Lords had a big war with creatures called The Vampire Lords who drained whole planets of their energy, but who, so far as I know, didn't have heralds on shiny surfboards. However, at the very end of time, the Earth becomes so polluted that the human race evolve into Haemovores who drink blood, dislike sunlight, hang out around graveyards, take boats to Whitby and recoil from crosses. And hammer and sickles. Hammers and sickle. (*) But that's no reason that fish people who drain the water from humans shouldn't have vampirey attributes as well, because the Doctor Who universe is a self-consistent fictitious cosmology which makes perfect sense. When the First Doctor met an animatronic Dracula at a futuristic funfair, he assumed that it was, not a haemovore or a vampire lord or a fish-thing, but that he was in an alien dimension where human nightmares had somehow come to life, and got a jolly stiff cease and desist letter from Universal Pictures a result. I'm sorry. What was the question again?
[*] Radio 4 newsreaders still say things like: "The MOD has announced that three soldiers will face Courts Martial...."