Either there are rules or there are no rules... I guess FR lore is so vast and extensive and so many people wrote stories in this world over so many timelines, that there has to be inconsistencies.i guess I assumed being a vampire in FR is similar to being a vampire in Buffy - you lose your capacity for good and become a (sophisticated) child with 100% id ruling you.

Larian's Biggest Oversight, what to do about it, and My personal review of BG3 EA
"74.85% of you stood with the Tieflings, and 25.15% of you sided with Minthara. Good outweighs evil, it seems."