Does it take that much imagination to assume that, if three vampires can masquerade in pretty high profile parts of society, there are other vampires in the game world as well? They don't necessarily have to be actual NPCs. I imagine there's more than twenty NPCs living in the Imperial City, but due to game limitations they can't put the thousands that would normally live in a city. If Bethesda had placed more than a handful of vampires throughout the game, it would look ridiculous. They're sneaky, they're not overpopulated.
The fact that there ARE vampire NPCs who are masquerading, and the fact that there ARE quests in the game that not only feature these NPCs but also feature as a distinct part of their character the very act of masquerading, DOES prove that they are developed. Less than you'd like? Sure. But if these guys are meant to be masters of illusion and deception, I don't want to be bumping into them, fangs bared, every night.
Yes, this is true. Jakben, Seridur and Janus are all vampires who have influence and power. But I think that there are numerous members of the Order hiding out in cities and such, maybe some even in the High Council!
Janus-Count of Skingrad(Sends player to wipe out barbaric vampires who made the Bloodcrust cavern ther abode. They breached Cyrodiil territory and he wants them dead)
Seridur-Leader of a vampire hunting faction(They hunt savage vampires. Makes sense for him to use people to kill the barbaric invaders)
Jakben-Masquerades as a noble. Has an unblemished history and mostly keeps to himself.