All 3 sub-genres can be classed under the same genre, the trouble is who gets top billing?
The classic movie definition of a zombie is the re-animated corpse that feeds off of living flesh.
The classic movie definition of a vampire is the re-animated corpse that feeds off of living blood.
And in folklore/mythology the classic definition of a ghoul is the re-animated corpse that feeds off of the living flesh and blood.
As someone who spends much of my time buried under files and books and other dusty things researching vampires in myth and movies; I have had to at times bring the stories labeled as "zombie" into the mix. Both film icons are considered viruses that are spread most often by one getting bitten. Both have the same surefire way to destroy the monster in question by decapitation.
Should vampire and zombie movies be relabeled under the same title of ghoul or given a new category all together?
Only the fans of each genre can say for sure.