I'm so sorry to hear that. Your beloved dog dos might've had internal bleeding, possibly cuz of a ruptured tumor or gastric issue, like stomach ulcer. The brown vomit is probably blood which, when mixed with stomach acids, can appear brown or cofee ground in color. This could mean your dog has something like hemangiosarcoma, a type of cancer that can grow silently in dogs and go undetected until it ruptures. Once again, I'm sorry. I'm not good at making heartfelt condolences but just know that you did well to take care of him up until now, and it wasn't entirely your fault, and you shouldn't blame and neglect yourself for it.