Vamping Vampire is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying, lifelink Plot Whenever you cast a spell while this card is exiled, put a stall counter on it. When this creature enters, it deals damage equal to the number of stall counters that were on it to target creature an opponent controls.