Can't drink each others' blood

When the vampaneze fell still, Vancha confronted Arrow. “You should
have accepted his name,” the Prince growled. “He faced you openly and
died bravely. He deserved to be remembered.”

“Drink his blood if he matters that much to you,” Arrow sneered.
Vampires could retain the memories of humans if they drained them of
blood.

“You know that we cannot drink the blood of a vampaneze,” Vancha
said.

Arrow shrugged sarcastically.

***

The Saga of Larten Crepsley: Palace of the Damned, Chapter 17