I wish they had fangs, it would be awesome x.x they don’t, sadly, you don’t see them at all when they talk. It could be as simple as adding night-elf-like ones. Never know!
At this point, a scenario like night warrior would be fine with me. Initially I wasn’t about it at all, but we’re getting an entire vampire ‘faction’ so my horses can be halted. As super cool as an allied race for San’layn would be, I see Venthyr as possibly being more likely, with San’layn ending up as customization. And where would we get the flavor abilities for them? Well… by siding with the Venthyr 
So yeah, that’d be fine. A night warrior scenario to touch upon the blood curse, say how you can be turned, maybe give some insight as to what they’ve been up to all these years, and how they’d end up joining Horde(could be as simple as ‘we’re breaking off from others of our kin and wanna help fight along side you’) would be fine.
We shall have to see. I’m really, really curious to see what Kael’thas has to do with this vampire faction. What could he look like? Would be be vampire-like or ghost like? Infused with fel still, or not? Regardless, I’m glad there’s stuff to talk about again relevant to this.
Edit: Highest covenant vote for forums is: Choosing a Covenant? Currently VENTHYR
Oh, so NOW people love vampires? Hahaha