Hel's plan as is has a decent Plan A and Plan B.

Without fast vampirism, Hel has no Plan B. When the Godsmoot measure does not yet pass, Greg is stuck there without any allies for 3 days. Then the Order cleans up the would be vampires, as they finally believe Belkar.

Plan B, rigging the votes of the dwarven elders, requires multiple allies, which absolutely requires fast vampirism. He needs an ally to seize the orb of teleportation while key members of the the Order are distracted by the moot. He needs an ally to take his place at the moot, while he himself travels to dwarven lands.

Without the orb of teleportation, winning the race to the council is nigh impossible.