Treantmonklvl20
Explorer
There are two ways to use Vampiric Touch + Grim Harvest:
1.) You can abuse it to regain HP between combats by killing inoffensive creatures like chickens (1 cp each!).
2.) You can use it during combat to try to tank. I have a PC who does this with his necromancer: plays him basically like a fighter. AC 15 (Robe of the Archmagi) + 4 (DX bonus, he's an elf with good rolled stats) + Shield spell = AC 19 or 24, which combined with the bonus HP from Vampiric Touch makes him a pretty formidable tank.
However, I think #1 is the primary usage.
I'll go out of order and start with 2: I'm of mixed opinion how formidable a tank using Vampiric touch is going to be for a number of reasons, but whether it makes you a good tank or not is irrelevant to my guide, as tanking is not a tactic the guide is built for.
1) OK, so you are casting a 3rd level spell outside of combat and the gains are that every chicken you kill you get 9hp back. I'll give you creativity points for sure. I do have some reservations still:
a) It seems like a trick to take advantage of a loophole that was never intended. I'm always leery of taking advantage of those as a player, and I'll definitely put a stop to them when I'm DM
b) I also have concerns about practicality. You are bring a bunch of live chickens with you while adventuring? How many are you bringing, how are you keeping them from running off, how are you keeping them from clucking and making a ruckus?
c) Don't expect to use them if the damage you took was from an area spell
My biggest concern is a).. Once again, points for creativity though (xp given)