Since my polymorph meme has only garnered three downvotes so far I thought I'd offer a bit more controversial take, and see if I can manage to stir the pot a bit with this one.
This is easily a rule as intended type of thing.
Not just that, it's also rules as written, Vampiric Touch explicitly states:
The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds.