I'm unsure to be honest. Surely you just mentioning Twilight means that subconsciously, you know that this is a cash in on how popular the vampire thing has gotten??
I disagree entirely. Daggerfall had vampires back in 1996, I haven't checked but it is entirely possible that Arena had them too. Vampires were already part of Skyrim but for some reason, they only gave werewolves a central place which I thought was wrong.
The mention of Twilight was simply because the general public immediately think of it if you say the word 'vampire'.
Of all the new areas they could have opened and explored, all the lore that they have at their disposal, all the things that are hinted on in the game (especially the Falmer thing) and they go with vampires, which is coincidentally, one of the most popular 'things' around at the moment? Hmmmm.
There are going to be at least two add-ons and this is just the first one. Why do the Falmer thing now? Better to wait a year or two.