This tactic is too action intensive. Unless you are casting the creature with a spawnpoint (in which case you would still be wasting your spawnpoint's action), you will have to use one full action to cast it and a second to attack it. If you intend to play a mage that wants to stay out of combat, it would be better to just include a few heals/minor heals instead of vampirism/sacrificial creature. Vampirism is also an inefficient source of pure healing because it heals only half the damage inflicted. The advantage to Vampirism over healing spells is that it rewards you with healing for attacking enemy creatures, which you want to do anyway. If you take away that aspect, it is inferior to dedicated healing spells.
Suppose you have a bitterwood fox you want to drain, and a melee attack of 5 dice. At most, you will heal 3 points of damage. For the same price as the fox, and a quick (as opposed to full) action, you could play a [mwcard=MW1I17]Minor Heal[/mwcard] and heal an average of 5 damage.
Vampirism should be cast on a mage only if that mage intends to melee its targets. Attacking an enemy creature is almost always a better idea than attacking your own creature. There are very few situations where healing in this manner would be worth the waste of your mage's action alone.