VE, CX, Flow all go through VRS and 'grep headers includes/libs/virtualrest/*' shows that user-agent is not being set there. This means that RESTBase then passes a RESTBase/WMF header in the user-agent string to Parsoid (In T129752#2118608, @mobrovac says that RESTBase adds that user agent string if the client doesn't provide any user-agent header).
So, if VRS can accept a user-agent param, then clients can pass RESTBase user-agent info via VRS and RESTBase would then pass that along to Parsoid which would simplify log analysis and attributing requests to the upstream clients that generated those requests.