Can I sacrifice the same creature twice?
The question is a bit unclear. If you're trying to sacrifice the Vampire Hexmage to pay for its own activated ability, and to satisfy the Plaguemaw Beast, then no.

For each Plaguemaw Beast and Vampire Hexmage, the act of sacrificing a creature is a part of the cost of activating the ability. If you fail to be able to pay the cost, the ability will fail to resolve.