Is a cannibal a vampire?

it depends. a classic vampire only drinks the blood of the victims. Now the new ones in the movies seem to eat the guts and other yummy things. so yes and no.

The defining trait of a vampire is the ability to steal life-energy. Now there is a whole spectrum of ways to do this, including eating the entire victim. But simply eating a victim doesn't qualify a creature as a vampire - only if there is life-energy transferred. And nutrition doesn't count as life-energy.