This is a pretty inspired idea! I haven't played much of Vampire Survivors or other games like it, but controlling a mob is an interesting concept that stands on its own. It's like you're coordinating a swarm instead of the "main character", and I dig it. The pixel art was appealing, and I was engaged in chipping away at the vampire's health. This is a pretty solid and polished game jam entry.
I will note that pressing keyboard keys to place objects on screen was a little awkward. I kept thinking "I have only two types of units; why can't I just left-click and right-click when I'm already using the mouse to aim?"
I also felt that I struggled to create a distraction with the knights as intended. I eventually figured out it's best to place them farther away and the vampire will prioritize walking to them. But his attacks felt too erratic, far-reaching, and unpredictable to make any meaningful strategy around this. Though maybe that just speaks of my unfamiliarity with these types of games?
Anywho, congrats on the successful entry!