- Units in Heroes 3

Undoubtedly, the intended meaning was the actual strength of vampire lords, that is, the combination of resilience with all other factors. A large group of vampire lords recovers better than a small one; therefore, 40 vampire lords are stronger than 4 squads of 10 vampire lords. Naturally, this applies to most situations, but not all.

This is only true if a fixed number of enemies are pitted against the vampires. That is, the "strength" of a vampire increases if, instead of 20 vampires, 30 vampires are placed against 100 halberdiers. In other words, you are again trying to move away from an objective assessment of the "strength" of units and towards the game situation.
However, if we consider the "actual strength" of vampires (without using game mechanics), it is still fixed. The "strength" or "resilience" does not change when (for example) there are 100 halberdiers against 20 vampires, or 1000 halberdiers against 100 vampires, or 10000 against 500, or (most importantly) 1 against 1. That is, the vampire will recover the same amount of HP that it deals, no more, no less. And when the stack size increases, with a fixed enemy stack, the vampire simply has a greater "gain" of life compared to the "loss." This, in fact, is also true for other ordinary units - the more of them there are, the harder they hit, the weaker they are hit in return, and the more of them survive. :)