Spoilers

All of the characters are weird, super humans with their own problems. At one point, the vampire freaks out and attacks the protagonist. Which leads to some plot ramifications. If you are to maintain that structure, you require a teammate to lose their cool at a pivotal moment when it has been established that the team leader was better and more logical than everyone else.

The in-universe vampire explanation gives a rationale for the scenario. This does not hold if you trivially swap out for a different character, so some kind of hard-wired trigger is required.