I have a single 6509 (VSS Sup) configured and running as a core switch. We are preparing to merge this into a VSS pair with a new 6509, fresh out of the box. I am comfortable with the VSS process but have always had two clean switches that I put into the VSS pair.
My question - if I am interpreting the dociuments correctly if I make my current production switch the Primary (higher priority) then its confiugration will be used in the merge. I am most concerned about all the VLANs, login credentials, existing portchannels when I execute the merge.
You don't need to give the current switch higher priority. If you configure this switch as switch one. it will be the primary (active) for VSS and the other 6500 (switch 2) will be the stand-by switch. I would make a very good back up of the config, in case some thing goes wrong. The primary switch config will be synced to the stand-by.