Playing on veteran mode this fight was super difficult for me - it was the sabre Grave Calling that made the difference, as it summons a grave imp when it kills a vessel, so you can keep boosting your numbers.
Using Wael's Wind on rest before the fight is essential to resist the charm attacks.
I didn't realise that you have to disable the AI control for your party to actually drink a potion - you have to turn it off, drink potion, then turn it on again - very annoying. But realising that helped!
The fight itself - stealth Aloth and Xoti up to the nearest vampire, the rest hang back to deal with the skeletons that appear. Aloth casts empowered missile salvo, Xoti casts hand of Berath - trying to weaken first vampire as much as possible. Then spam summons - i used a lot of figurines. Chuck a wall of many colours down between you and the summons vs vampires fight going on. Kill any undead that cross the wall, cast weaken spells from across the wall on the vampires. I was eventually down to my Main and Pallegina vs 3 vampires. the Unrelenting skill saved me in this case... otherwise would prob not have made it.
