The purpose of this study is to determine if vimseltinib is safe, tolerable and works effectively to treat adults with active moderate to severe cGVHD. Participants will be treated with vimseltinib in 28-day treatment cycles for approximately 2 years.

Inclusion Criteria:

1. Must be allogeneic hematopoietic stem cell transplant (HSCT) recipients with moderate to severe cGVHD requiring systemic immune suppression.

   a. May have persistent active acute GVHD (aGVHD) and chronic GVHD (cGVHD) manifestations (overlap syndrome).

2. Participants with active cGVHD who have received and failed at least 2 prior lines of systemic therapy.

3. Stable dose of systemic corticosteroids is permitted but not required. If being taken, participants should be on a stable dose of corticosteroids for at least 2 weeks prior to starting study drug treatment.

4. Adequate organ and bone marrow functions.

5. Participants of reproductive potential agree to follow the contraception requirements.

6. Karnofsky Performance Scale (KPS) of ≥60.

Exclusion Criteria:

1. Has aGVHD without manifestations of cGVHD.

2. Prior use of colony-stimulating factor 1 receptor (CSF1R) inhibitor for cGVHD.

3. History or other evidence of severe illness, uncontrolled infection, or any other conditions that would make the participant unsuitable for the study. All wounds must be healed and free of infection or dehiscence.

4. History of malignancy except for:

   1. Underlying malignancy for which the transplant was performed
   2. Malignancy treated with curative intent and with no evidence of active disease present for more than 3 years prior to enrollment and felt to be at low risk for recurrence.

5. Malabsorption syndrome or other illness that could affect oral absorption.