To increase your vampires generation you have to diablerize a vampire with lower generation. Meaning your 11th gen vampire must diablerize a vampire with 10th generation or lower.

I'm having a similar problem where my vampire has attempted diablerie on a vamp of lower gen, lets say mine is 11 while the vic is 10 in case it matters, but the fight only ever kills the vampire I'm trying to eat.

I've learned that the best way to do it that works is to capture target vampire, administer anesthesia and then do the diablerie.

Yeah bro, used to have problems too, here's a few tricks I picked up.

1) When your vampire does Diaberie he'll run up and drink all the blood out of the other vampire. If he ain't doing that then something's going wrong.

2) Like Lord Shalltear said, you need to diaberieize a vampire of a lower generation.

3) Hey here's a big tip to make your life easier. If you can get a psychic shock lance (or one of the items from Hastur in the Rims of Madness Cults mod) then you can incap basically any generation of vampire, making them a free meal.

Lower as in smaller number?