I didnt further test this and this is just what i thought happened:
When using vampiric weapons the heroes dont realize there is damage taken somehow and wont start healing you. Reason for the rit healing you first could be that the aoe heal spell is a 2 instance skill (casting the ash and dropping it) and will automatically drop at some point eventhough the hero doesnt notice the healing is actually needed.
Dont have a clue about the rest of your questions