Dragons are needed to eliminate Mages and reduce enemy morale by 1, as well as for overall defense.

Well, you can also use them to block enemy archers, if you can manage it... :) Despite their low hit points, they often draw a lot of attention, like a scarecrow: enemies will try to eliminate them at all costs, even humans. Consequently, Vampires take much less damage.

I don't see much point in making Mages the main focus. Unless it's an emergency, when there are no crystals or dragon stones nearby, and therefore no way to build Vampire Lords.

I don't understand the appeal of using Mages... Battles that Vampire Lords can easily win without losses become less advantageous when replaced with Mages: Skeletons suffer losses, and Mages themselves might take damage if enemies reach them in time.
In this case, you could probably combine Mages with Vampires, but that's only if you have nothing else to do and too much money.

Or, more profitably, build a second army with Mages and Knights, and use it to attack another direction. However, I consider this option risky: given that Necropolis armies move across the map at the speed of a wounded Estonian turtle (due to the Skeletons), there's a high chance of running into an enemy and not being able to escape in time...