In Morrowind, it certainly seems so, as guards will attack on sight. But is this something unique to the province similar to the illegality of Necromancy? As certain characters - including Mages Guild members, who are imperially chartered, were non-plussed by Vampirism as they were Necromancy..
However, in Cyrodil, all Vampires the player is quest-required to kill have generally murdered someone - Seridor for example. So is Vampirism 'legal' in the same way Necromancy is?
Whilst Count Skingrad is not publically known as a Vampire, the Empire's ruling class seems comfortable with it provided he does not feed.
In-game books reference Vampire Hunters, and some can be met in game, and we can infer from Raynil Dralas' murder in Bruma that it is accepted practice - but, again, the 'vampire' in question was known to have murdered someone.