This study is testing a drug called **vimseltinib** for people with chronic Graft-Versus-Host Disease (cGVHD). cGVHD is a condition that can happen after a person gets a **stem cell transplant**, which is when healthy cells are moved into your body to help replace damaged or missing ones. The study aims to see if vimseltinib is safe and effective. Participants will take the drug in 28-day cycles for about 2 years. **Eligibility**: Participants must have moderate to severe cGVHD and have tried and not succeeded with at least two other treatments. They must have stable health and organ function. **Exclusion**: People who have only acute GVHD, have used certain other drugs for cGVHD, or have severe illnesses can't join. **Key Points**: - About 2 years of participation, with 28-day treatment cycles. - Must have tried and failed at least two other treatments for cGVHD. - Certain health conditions and previous treatments may exclude participants. This study is important for finding new ways to help people with cGVHD.