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.
There are rukes but rules existing creates dramatic opportunity to break them. So vampirism is inherently corrupting but because of that there will always be hero characters who stay pure/fight their evil selves by sheer force of will or self-sacrifice. Because those are more interesting to read and write about and writers are suckers for drama.