Probable antitoxin component of a type II toxin-antitoxin (TA) system. The cognate toxin is VapC5. Forms a complex with VapC5. The C-terminal domain (residues 55-86) does not neutralize non-cognate antitoxin VapC4; exchanging the N-terminus (residues 1-54) with VapB4 however allows neutralization of VapC4, while exchanging the C-terminal domain (residues 55-86) does not. Belongs to the phD/YefM antitoxin family.