So you gotta pay big to get in, for Harkon it was 1000 innocents and for his daughter it was [censored], and then to get power you have to infect others with vampirism, gives you a thrall and I imagine extra powers and Molag Bal has a pretty new soul(cause sanguinare vampiris kills you). Then your thrall does the same to another mortal and then they become vampires and then they infect others...

Sooner or later all of Tamriel has a huge vampire population and Molag Bal coudln't be happier that he has so many souls to call his own.

Bal created vampirism to spite Arkay. I doubt he cares about individual vampires enough to bother.

He would care about the power he can harvest from the souls the vampires helped him enslave