Two Options.

He recognized that he wasn't prepared and powerful enough to defeat the army, so he didn't attack them. But he also knew that the longer the fight between him and the Yggdrasil army dragged on, the higher his chances of defeat would be, so he knew that he had to do something to solve the matter of inadequate preparation and power as soon as possible.

The vampire civilization likes to drag things out too. The more time they are given, the more of the inhabitants of the world they can turn and enslave, and the more soldiers they can have.

But he has heard the older vampires talk incessantly about how it is a bad idea to let the Yggdrasil civilization settle down. He heard that the humans were too smart and insidious.

He doesn't think humans are smarter than vampires, but what he thinks cannot change the fact that the vampires have suffered more defeat than losses at the hands of the Yggdrasil civilization since they first made contact with one another.