If we are speaking of Vlad, the Empire may honestly almost be better off than before.
For one, less taxes :D
In general religion would more or less be abandoned in an organised form, civil wars should be a thing of the past/ nowhere near as prevalent, as the man on the throne is not only immortal but also immensely powerful.
Socially speaking we'd see what happens every time when vampires build an empire, they are on top, mortals are treated in a more meritocratic manner than before.
Knowing Vlad the entire affair would be more stable than Ushoran's little project back when.
However, as Vlad is the Chadpire, he will one day decide that Bretonnia looks fairly tasty as well, and while he's at that Estalia as well.
Basically (for the Empire) things would arguably go fairly well.
His biggest problems would most likely be the Skaven (since he'd singlehandedly unite the surface world and they have a history of killing off necromancers with too much power) and Chaos, since the midnight aristocracy has no love for the Dark Gods.