A Study of Pembrolizumab/Vibostolimab (MK-7684A) in Relapsed/Refractory Hematological Malignancies (MK-7684A-004, KEYVIBE-004)

Inclusion Criteria

• Inclusion Criteria - Have confirmed relapsed/refractory classic Hodgkins Lymphoma (cHL), Primary mediastinal B-cell lymphoma (PMBCL), Follicular Lymphoma (FL), Diffuse large B-cell lymphoma (DLBCL) or Non-Hodgkins Lymphoma (NHL), or multiple myeloma (MM). For PMBCL, DLBCL, FL, and MM: - Must be relapsed or refractory to CAR-T-cell therapy or unable to receive it. For DLBCL and NHL: - Must have exhausted or be ineligible for or intolerant to all treatments, which in the opinion of the investigator are standard of care for their disease. For NHL: - Participants with Mantle cell lymphoma (MCL) must have received prior Bruton's tyrosine kinase inhibitor therapy. All participants: - Have measurable disease. - Have adequate organ function. - Participants who are Hepatitis B surface antigen (HBsAg) positive are eligible if they have received Hepatitis B (HBV) antiviral therapy for at least 4 weeks and have undetectable HBV viral load before allocation. - Must be able to provide newly obtained bone marrow biopsy or aspirate material for disease assessment. - Female participants are eligible to participate if not pregnant or breastfeeding, and at least one of the following conditions applies: Is not a woman of non child-bearing potential (WONCBP) OR Is a woman of childbearing potential (WOCBP) and using a contraceptive method that is highly effective, or be abstinent from heterosexual intercourse as their preferred and usual lifestyle. Exclusion Criteria For DLBCL and NHL: - Has lymphoplasmacytic lymphomas, Waldenstrom's macroglobulinemia, chronic lymphocytic leukemia (not associated with small lymphocytic lymphoma), Burkitt (-like) lymphoma, mature T cell and NK cell neoplasms, immunodeficiency associated lymphoproliferative neoplasms, or histiocytic and dendritic cell neoplasms. For MM: - Has oligo-secretory myeloma, plasma cell leukemia, smoldering multiple myeloma, or monoclonal gammopathy of undetermined significance. - Has a history of primary amyloidosis, hyperviscosity or POEMS syndrome (plasma cell dyscrasia with polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes). - Has known prior or current central nervous system (CNS) involvement. For Epstein Barr virus (EBV) positive DLBCL: - Associated with a solid organ transplant. For all participants: - A WOCBP who has a positive urine pregnancy test within 72 hours before study intervention allocation. - Has clinically significant cardiovascular disease within 12 months from first dose of study intervention. - Has a history of a second malignancy. - Any PMBCL participants that require the use of urgent cytoreductive therapy. - If the participant had major surgery, the participant must have recovered adequately from the procedure and/or any complications from the surgery before starting study intervention. - Has received prior radiotherapy within 2 weeks of start of study intervention. - Has received a live or live-attenuated vaccine within 30 days before the first dose of study intervention. - Is currently participating in or has participated in a study of an investigational agent or has used an investigational device within 4 weeks before the first dose of study intervention. - Has a known severe hypersensitivity to MK-7684A, vibostolimab or pembrolizumab and/or any of its excipients. - Has a known history of Human Immunodeficiency Virus (HIV) infection. - Has an active autoimmune disease that has required systemic treatment in past 2 years. - Has an active infection requiring systemic therapy. - Has a known psychiatric or substance abuse disorder that would interfere with the participant's ability to cooperate with the requirements of the study. - Has present or progressive accumulation of pleural, ascitic, or pericardial fluid requiring drainage or diuretic drugs within 2 weeks before enrollment. - Has dual active HBV infection (HBsAg (+) and /or detectable HBV DNA) and Hepatitis C (HCV) infection (anti-HCV Ab (+) and detectable HCV RNA) at study entry.. - Has had an allogenic hematopoietic stem cell/solid organ transplantation within the last 5 years.