Well, a vampire is useful against any living unit, except perhaps against Crusaders, and maybe against magical creatures...
Without a hero, any army loses its strength, especially without a mage or tactician, and becomes similar to a neutral one. If you're facing a strong player, you won't be able to make effective moves with your units.

I was talking about the army in general. For example, Life can take a ballista on level 2, and level 3 is all about defense against Death, so vampires will have a very hard time in that case. And when Order builds golems and mechanical dragons, vampires often nervously gnash their teeth on the sidelines. They can't do anything against undead. But they are indispensable in fighting against Nature (if it doesn't have elementals) and Chaos. I agree that an army with a hero is better than one without. Even psychologically, it feels more natural. As for mages, it depends on the mage. A Sorcerer is better off running solo - you can level up magic resistance and silence everyone with Armageddon. But a Priest definitely needs an army. Oh well, I've been playing "The Coming Storm" and now I'm thinking not about which monster is more useful, but about which hero is cooler :D