Antitoxin component of a type II toxin-antitoxin (TA) system. Upon expression in M.smegmatis neutralizes the effect of cognate toxin VapC15 (PubMed:20011113). Partially inhibits the RNase activity of VapC15 (PubMed:25450593). The heterotrimer binds 1 Mg(2+)-Mn(2+) pair while the heterotetramer binds 2 pairs. Both metals are shared by the toxin-antitoxin pair. The heterotrimer binds 1 Mg(2+)-Mn(2+) pair while the heterotetramer binds 2 pairs. Both metals are shared by the toxin-antitoxin pair. Crystallizes as a VapB15-VapC15(2) heterotrimer and as a VapB15(2)-VapC15(2) heterotetramer; each toxin pair forms a homodimer which creates a channel in which the antitoxin binds. Induced by hypoxia.