Vampires are banished because of past problems, which created prejudices and fear.

When they were discovered, vampires were terrifying creatures. They drank human blood, turning their victims into more vampires and killing them. People associated blood drinking with demons and revenants. Naturally, without defenses against the creatures, they became hysterical. (Suspected) vampires were murdered and buried with sharp objects so that they would be impaled if they were to return.

As time passed, vampires became less and less common. They had been killed off. There were no reports of vampires in one area for years.
Then, in that same area, a group of vampires were found in the outskirts of the town. They had not been threatening to anybody, but were brought upon the ruler of the town (king, queen?). Because the vampires had not killed or harmed anyone in anyway, the ruler could not condemn them to death. That was not the way to rule.
(The vampires still having emotions and feelings. They did not harm anybody because they knew they would be murdered. They drank animals blood)

Therefore, the vampires were cast outside of the town, left on their own. As time passed, this banishment remained. The people did not believe that they could come back and do no harm, even though they had not done any before.

Vampires were solely banished because of fear. They were feared once, then forgotten, then feared again because they had been banished by a cowardly ruler.

Vampires, why they're cast out