True, but you're still getting unlimited units Mind/Body/Death III, sorceror/summoner, maxxed combat strength / march move / bonus movement, and whatever else your heart desires, all from one side of their hybrid, and since you're recycling your vampire lords due to their immortality you might at well get them to 20th level or so. My favorite simple mix is to buy up the city assault tree with the Vampire Lord and splice with a Shadow.
If you want unlimited immortals, go with cauldron/altar/forge and sacrificing immortal units. You get nice production and research bonuses, and every sacrifice nets you an Infernal Lord (who is also immortal, and the cauldron isn't limited to only 3 infernal lords). It does, however, require that you get 2 specific wonders in one city, which may make it prohibitively difficult for you in MP.