Additional Study Information: The main purpose of this research study is to investigate if a drug called Vedolizumab is safe and effective in preventing acute graft-versus-host disease (attack of donor cells on recipient healthy organs after transplant) of the gut in comparison to placebo (dummy drug). This will be studied in patients undergoing Allogeneic Hematopoietic Stem Cell Transplantation (allo-HSCT) from an unrelated donor for a blood cancer such as acute leukemia, chronic myelogenous leukemia, myelo fibrosis, or other myeloproliferative disorder.