I don't know how you play, the Mage keeps defeating me every time, as soon as he appears after a few turns.

It's a controversial thought, of course, but try playing on Recruit difficulty. I don't plan to further strengthen the undead for now; otherwise, it will be like bullying an AI baby.

The spell is in the book, but it's not active in battle.

Ah, well, if it's only in battle, then it's a different matter. In that case, I have nothing to add to PingWing's comment above.

Will Vampire Guards join you anywhere else besides the portal?

I answered this question negatively before, and that's correct. Besides the portal, they don't join anywhere else (in the form of stacks on the map), but the stack near this portal is waiting to join the player's army.