Cheapest priced copy (Duel Decks: Sorin vs. Tibalt): $0.35 at Card Kingdom, near mint, their price list as of .
Sengir Vampire 
Creature — Vampire · 4/4 · Uncommon · Revised Edition · #129
Flying (This creature can't be blocked except by creatures with flying or reach.)
Whenever a creature dealt damage by this creature this turn dies, put a +1/+1 counter on this creature.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
37 printings · $0.35 to $799.99 at Card Kingdom
3 rulings from Wizards of the Coast
- 15 July 2007If Sengir Vampire deals nonlethal damage to a creature and then a different effect or damage source causes that creature to be put into a graveyard later in the turn, Sengir Vampire's ability will trigger and it will get a +1/+1 counter.
- 22 September 2011Each time a creature is put into a graveyard from the battlefield, check whether Sengir Vampire had dealt any damage to it at any time during that turn. If so, Sengir Vampire's ability will trigger. It doesn't matter who controlled the creature or whose graveyard it was put into.
- 22 September 2011If Sengir Vampire and a creature it dealt damage to are both put into a graveyard at the same time, Sengir Vampire's ability will trigger, but it will do nothing when it resolves.