Inclusion Criteria  - Have confirmed relapsed/refractory classic Hodgkins Lyphoma (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 macroglobulinema, 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 a history of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease. 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.