As far as I know, the VCS integration is managed by the VDP server, that’s why the Windows server user is used when unchecking the *“Use user and password”* checkbox.
I think it is not possible to set automatically the username in the commit message. In your situation, if you want to know which user performs the commit, I would use the alternative of having one local VDP server for each user so each one commits the VQL to the TFS server with her username. If you want to use only a VDP server I would create one database per user and configure the VCS integration per database using the username and password.
Hope this helps!