The spell description says 1d6 dmg for every 2lvls up to 6-36dmg which the caster gains on top of his current hit points....
What actually happens?? Except that dmg is never so little let me mention this :
I managed to kill an ogre magi in the copper coronet with just one vampiric touch...
Today i cast it on a monster with magic resistance... Of course no dmg was dealt..but guess what i got +70 hit points....so i then had a mage with 150 hit points or so...the spell should just fizzle on magic resistance not work at its super max!! i cant believe u can get 70hit points or kill anyone with this unbelievable spell which actually should be banned from the game....3rd level spell and deals 50dmg at least, no save...

As much as i love it i hate it...
I thought there was a baldurdash fix about this...but i doubt if its implemented in the bp...