Mindless Null can't block unless you control a Vampire.
Only a bloodchief can create vampires. The rest sire nulls, who become mindless beasts in their former bodies.
Official Rulings for Mindless Null
10/1/2009 : Whether you control a Vampire matters only at the time you declare blockers. Once Mindless Null blocks a creature, that won't change, even if all the Vampires you control leave the battlefield.