Yeah, the humor didn't do much for me, either. It was a little too much, IMO, especially with the Avis joke being beaten to death. The humor worked for me better when there was less of it and it came out of nowhere (i.e. just as you've been lulled into taking it as serious sci-fi, a joke comes).
I was a bit surprised by the dark turn at the end, too. It catches you off guard, since you expect the episode to end happily, like most Star Trek episodes. It actually gets you to think.
Space vampires have been a recurring element in science fiction for over 50 years. There were numerous movies about them in the 60s, there was a popular novel named The Space Vampires in 1975 that was turned into the 1985 film Lifeforce and there was an episode of Buck Rogers in the 25th Century (1980) about them: