Bloodthrone Vampire

• WWizards of the Coast2010-06-15
  Official Ruling
  If you sacrifice an attacking or blocking creature during the declare blockers step, it won't deal combat damage. If you wait until the combat damage step, but the creature you wish to sacrifice is dealt lethal damage, it'll be destroyed before you get a chance to sacrifice it.
• WWizards of the Coast2010-06-15
  Official Ruling
  You can sacrifice Bloodthrone Vampire to activate its own ability, but it won't be on the battlefield to get the bonus.

Rulings via Scryfall