Inclusion Criteria:  Male and female subjects ≥ 18 years of age undergoing HSCT.  Planning to undergo allogeneic hematopoietic stem cell transplantation from a human leukocyte antigen matched sibling, haploidentical related donor, or HLA-matched unrelated donor with either bone marrow or peripheral blood stem cells as a graft source, and with any conditioning regimen  Exclusion Criteria:  Has severe colitis of any etiology or active/currently-treated inflammatory bowel disease (IBD) or total colectomy.  Evidence of relapse or progression of hematologic malignancy (minimal residual disease is allowed).  Transplant using umbilical cord blood or ex vivo T-cell-depleted HSCT  Receipt of chimeric antigen receptor T-cell (CAR-T) therapy.  Received a fecal microbiota transplant (FMT) or any live microbial therapeutic within 3 months prior to Screening.  Known allergy or intolerance to oral vancomycin.  Concomitant participation or participation within 1 month or 5 half-lives of another investigational treatment.