See, I went back and read the Malack scenes, and I'm still not getting any of what you are saying. The fact that Malack is specifically hiding the fact that he is a different spirit is totally in character. Think; if you were a spirit from the negative energy plane, and you knew that your master used said spirits, rare as they are, to accomplish important tasks, would you want to be revealing that fact to the world? No, you would keep it hidden, if not actively work to suppress any information on that from leaking out to the world.
As for rarity the thing that determines if the knowledge is known is the rarity of vampire
resurrections, not of vampires themselves. That means you've got to multiply the rarity of encountering a vampire (of which we have only encountered one in the whole journey), with the chance of someone who would actually be resurrected being vampirized (I'm guessing the majority of vampires fall into the "kill-on-sight" category for many adventuring parties rather than the "oh let me help you" category), and then by the factor of needing a cleric powerful enough to actually perform the resurrection. Combine that with a potential PR modifier for vampires due to them often being rather high-profile villains, and it's totally possible that vampires could be commonly known about but that the knowledge of the actual mechanics within them (which vampires have been actively suppressing) is totally unknown. Who knows, maybe the top level clerical organizations
do know and are just suppressing the fact so that some vampires will stay with their parties and end up being forced into resurrections rather than all of them immediately going to join team evil because everyone knows they aren't the same person.
As for not knowing exactly how everything works, that's totally a common thing in any fantasy series except for ones that highly prescribe to the
Magic A is Magic A Trope. The only difference here is that because his system is based off of one that is a Magic A is Magic A it throws people for a loop when he reveals rules in his system that aren't identical to the base one. The fact that he doesn't necessarily feel the need to explain what those rules are is also a totally valid thing in a story. IMO it would be weirder if Malack and Durkon had suddenly stopped and had a discussion on how common vampires were and how they were known about worldwide before trying to kill each other than anything else. Of course another alternative would be to use out-of-story pages to explain something, but I don't think that's particularly needed in any given story, unless the author both wants to have a Magic A is Magic A world and they feel the need to explain those rules to their readers.