I can't speak for WotC, but I think the narrow options for the vampire was intentional. The class is a way for a character to showcase his full vampiric might during combat. He's not a "wizard vampire", casting spells and occasionally using vampire powers (that type of character can be achieved with the Vryloka).
If you look at the Buffyverse, there's not much to differentiate Angel, Spike, Darla, or even the Master. Sure, you have the Drusilla with some psychic abilities, but that can be achieved by, say, making her a kalashtar vampire.
That being said, if you can come up with a cool concept for a vampire expansion (by making, say, a pennangalan or hopping bloodline), go for it.
Oh my God you should so write that as an article for dragon, especially the pennangalan vampire. I'm totally serious, that would be so cool, a pennangalan build for the vampire.
Why so many Arcane paragon paths compared to everything else?
Btw way I loved the way you made Vryloka different from true vampires and dampyrs. In White Wolf terms the Vryloka are like rogue ghoul family's with the Red Witch as an Asnam as a secret patron.