So what are your lore-y takes on why no one notices or cares that you are a vampire until stage four?
). Here's a fragment thar might be the answer to your question:
It's just that the PC is not indistinguishable from mortals when they are a vampire. They look pretty different. Of course one can always imagine that they don't look like a vampire, but in the game you clearly do.
At least it's better than when you're seen as a werewolf in Morrowind- you could be seen in a werewolf form, leave, come back few days later in a human form, but people would, for some odd reason, know you were that werewolf that attacked them long ago. Don't know how it works in Skyrim since I have never turned myself and went into town or someplace like that.