After seeing the dragon dive under the water I'm not really worried about the vampires. If a dragon can go under and then pop back up to attack the player, I'm sure a vampire could do the same thing quietly.
I'm actually thinking that may be how you find them.
Someone else suggested that's the entrance to the Volkihar castle. As in, perhaps a town requests that you, Dragonborn, come kill this dragon that's terrorizing him, then during your fight he dives under the ice and you give chase, and before you know it you've stumbled across the Volkihar.
Does it make perfect sense? Well no, that doesn't explain how you get to the castle since it's clearly not underwater, but from a lore perspective this would be a lore-friendly introduction to the Volkihar.
Why do arrogant peons assume we should already know about everything that isn't a recent development in the world?
Here's a big reason it wouldn't work for PC vampires, and secondly there is no established lore on Volkihar, a single book written by a vampire is not reliable and can not be considered established until backed up by another source or seen with our own eyes. A Vampire has every reason to lie about the abilities of other vampires.
Then why does this book exist?
I don't know where you learned how to write, but any classes I ever visited on writing say you don't write without purpose. Say for example you're reading Twilight (lulz the only book that comes to mind at the moment, what with all the vampire vs. Werewolf here on the forums), and in the middle of the book FOR NO APPARENT REASON, there's a chapter dedicated to the story of Bella's grandfather and the time he served in WWII. Just some random ass chapter about his service in WWII that has nothing to do with anything.
Would this be a good idea? Absolutely not. It makes no sense, serves no purpose and doesn't match the theme of the book at all. You're implying Immortal Blood is the same. While you could use that as an excuse to justify the Volkihar as they are now, that does NOT justify Bethesda's writing skill or the fact that it was stupid as hell to include that book if it's unreliable and completely false.
There seems to be a fair bit of vampire lore in the vanila version of Skyrim. I just think it's a bit lame they'd make it a theme of a dlc pack for a TES series. It's like the dovakiin was destined be a hunter of some sort. Dragon hunter, bounty hunter, witch hunter, werewolf hunter, now even a vamipre hunter.
Actually it looks like he's intended to be a vampire, which admittedly would be kinda cool. That at least ties into the theme of the dragon blood driving people power-hungry.