"Like being a werewolf in the Companion quests, you transform into the Nosferatu-like vampire lord by pressing the right bumper. Once you take this dastardly form, you can make devastating melee attacks or cast spells from afar. The vampires are given a few unique powers as well. Vampire Servant allows you to reanimate a low-level corpse to aid in your battle for 60 seconds, and Vampire Sight grants you night vision for 60 seconds. You can also use vampires tiny bony wings to fly for a short period of time by hitting the left bumper." -Gameinformer

So, obviously as it says, it's like the Werewolf transformation. Now, it didn't say it was like the transformation in every way. (As in the whole once per day thing)

Now, this quote tells us something very useful and important: "Once you take this dastardly form, you can make devastating melee attacks or cast spells from afar."
From this it has been proven you can use both normal and Vampire like spells while in this state.

Next quote: "You can also use vampires tiny bony wings to fly for a short period of time by hitting the left bumper."

From this quote you can tell (For all of you who hate the wings) that you don't NEED to use the wings, it's all optional. But at the same time, you can't use the wings forever.

Now, I don't know about you guys, but some of my questions have been answered by just this one paragraph. The only questions I have left are: "Can we transform at will to the point where we don't have a whole day cooldown?" and "How exactly (And when I say exactly, I mean every inch of detail) do the new perk trees work?" As far as I know, all you do to "gain" the perks is by mauling or feeding.

From the recent video, we can tell that the form has melee and magic.

The wings do NOT seem to be "optional", just the option to actually take flight or not.

There are at least 5 other threads that cover all these facts. They may help shed some more light on your questions.

http://www.gamesas.com/topic/1379455-new-dawnguard-info-2/

http://www.gamesas.com/topic/1379431-gameinformer-hands-on-new-info-note-this-is-note-the-leaked-info/

Well, that's what I meant by optional wings, you don't need to use them.

Please use the existing topic about the game informer article.