I just updated from the base Skyrim game for the first time. Woohoo. Behind the times, I know. One thing I was looking forward to was being able to play a stage 4 vamp without upsetting the town folk. In a funny irony, though, dawngaurd vamps are hideous and it should be quite obvious to everyone they are vamps.

I can play stage 4, but I lost my seductress. Alva gets to still be cute. Why can't my characters?

She's a Cyrodiilic Vampire. As long as they're well fed, you don't distinguish them from normal people. Only the Detect Dead spell, will reveal her "condition".

The ingame book http://www.uesp.net/wiki/Skyrim:Immortal_Blood explains it all.

No, like Seridur in Oblivion, she's given a subtler appearance so that her condition isn't immediately perceivable, which would make the quest's outcome obvious. Given that in Oblivion, every vampire was a Cyrodiilic vampire, yet Seridur had an apparently unparalleled and unique ability to hide his condition, I'd chalk it up more to game design rather than lore implications in both his and Alva's case.

If my memory serves me well, my vampire characters in Oblivion looked alright when fed -not exactly what I had finalized during character creation, but quite passable.