He was the son and successor of Otto and of Bertha, daughter of Ulric Manfred II of Turin.

As his first wife, Boniface intended to marry an unnamed woman who had been betrothed to his brother, Anselm, before his death.

In 1079 Pope Gregory VII commissioned the bishops of Turin, Asti and Alba to convince Boniface del Vasto not to marry the woman, because her betrothal to his brother created a relationship of affinity between them.

[4] Boniface and Agnes of Vermandois had: Boniface was also the guardian of his brother Manfred's children: Henry del Vasto, who married Flandrina, a daughter of Roger I of Sicily, and Adelaide del Vasto, who was Roger's third wife.

Two of Manfred's unnamed daughters married Roger I of Sicily's sons from previous relationships, Jordan, Count of Syracuse, and Geoffrey, Count of Ragusa (who may have died before the marriage actually took place).