He implantation followed by thermal anneal is a well-established technique for creating layers or bands of cavities in silicon. This process is a consequence of the interaction between He and ion-implant-induced vacancies. Applications of such cavity layers include gettering and localized minority carrier lifetime control, and compliant substrates for lattice-mismatched heteroepitaxy. Studies have shown that the presence of interstitial-type defects can lead to the shrinkage of He-cavities due to the interstitial capture by the cavities. However, few of them deal with the interaction of cavities with vacancies. Here we present results on the formation of He-cavities in Si in the presence of atomic hydrogen and vacancies produced by effusion of hydrogen. Following a helium implant, samples were hydrogenated with an electron cyclotron resonance (ECR) hydrogen plasma. Control samples without any hydrogenation were also used. We studied the influence of hydrogen on void morphology. While hydrogen enhances void size at higher energy implants, the enhancement effect is absent in lower energy implants. The results underscore the role of vacancies in void formation and growth.