Vampirism is a religion?

Well, yes. But if you're referring to the scene where Van Helsing is explaining the situation to Father Stepnik, keep in mind a few things. First, as the story was taking place late in the 19th century, even an "advanced" scientists such as Van Helsing still had some misconceptions to shake off. Second (and this is where I mainly come from), Van Helsing may have been trying to soft-pedal his explanation to Father Stepnik: wanting to emphasize the historical aspects of vampirism (as he saw them), rather than hit the priest over the head with the more monstrous aspects.

"I'm not reckless . . . I'm skillful!"

It's somewhat explained better in the Hammer film, Kiss of the Vampire, three years later.

In fact, that movie gives some context to The Brides of Dracula, since both take place in Germany (not Transylvania)and both refer to a cult that requires recruitment of thrill-seeking youth as new members.

I think that he means that those drawn to the occult, intentionally defying christianity or not, are most susceptible to it because they seek it out.

The cult of vampirism is a recurring theme in the early Hammer Dracula/vampire films. Why would you think it was stupid?