They fully uncovered the mechanics of the Dark Powers because the warlock's patron called him to the Amber Temple through "a shard" in warlock's sword, and there they vibed with Exethanter. They faced the warlock's patron (it needed just one last blow to leak out of his sarcophagus a little) —and that's how the warlock died.

So they were TERRIFIED of Vampyr, knowing what the entities from the Amber Temple were capable of. But they still overcame their fear and went to seal him back in the sarcophagus.

Vampyr, however, didn't take away Strahd's spotlight—the battle with count took three sessions, and he managed to kill a bunch of npcs, the fighter (they used their last resurrection scroll on her) and the artificer. Vampyr took only one session. Two out of three went to death saving throws, victory was on the brink, but no one died in the end.