In any game or setting with so many monsters there will tend to be overlap between types. Illithids do take some cues from vampires, but their role is being the face of the alien in D&D.

Unfortunately the Aboleth does this just as well, and is more alien.

Any D&D setting will have to chose what monsters it includes, and normally settings will go along with a theme that isn't 'anything and everything'. In general a setting that focuses on the alien will have mind flayers but won't have vampires, while a setting that focuses on corruption will have vampires but won't have mind flayers. Having two redundant creatures is not as massive a problem as it seems.

Now other games tend to have smaller 'core' bestiaries, and will add more monsters in per setting. Fantasy AGE has a relatively small set of monsters with little overlap. Savage Worlds has more overlap but in many cases setting bestiaries are replacements for the core bestiary. Lamentations of the Flame Princess doesn't have a 'standard' bestiary and recommends making all monsters specifically for the adventure.