Trial of Prophylactic Versus Empirical Vancomycin for the Prevention of Streptococcal Sepsis After Hematopoietic Cell Transplantation

The primary objective of this study is as follows: * To compare prophylactic with empirical vancomycin administration for reducing early viridans streptococcal bacteremia in allogeneic hematopoietic stem cell transplant (HSCT) patients. The secondary objectives of the study are: * To examine the safety and tolerability for each vancomycin administration approach. * To measure the incidence of vancomycin-resistant enterococcal (VRE) infections for patients managed with each of the two vancomycin administration approaches.