The fact that the vampirilis can't have access to planetary improvements makes them unable to get the very powerful improvements that increase the ressources yields for deposits of luxury and strategic resources. They would need an ark support system to balance this i believe.

Also to balance the arks i think they shouldn't have the same stats while they are anchored and while they are as a fleet (when they are anchored they tap into the system's ressources increasing they power a lot while when they are used offensively they should be a lot weaker)