Dick,
I'll answer your question.
Separation is either lateral or vertical, if the procedural ATC was unsure of the VFR position, due to poor position reporting, ATC would use vertical separation and only allow the IFR aircraft to descend to 6000', he would then ascertain the DME distance, Radial and/or HDG of the VFR traffic and use Air Traffic Controlling to create lateral separation from the IFR aircraft. Too many options to list, but to keep it simple in my example, think of pure DME separation, i.e. VFR remain outside xxDME, IFR cleared to descend once inside xxDME, with the required buffer
Trust me procedural ATC have many tools and won't sacrifice vertical separation until lateral separation is assured.
So in summary, once ATC had created positive lateral separation the IFR aircraft then descends through the VFR aircrafts level.
In most cases this is easily achieved, but I have also had to enter a hold overhead a radio aid and descend in the hold before commencing a procedure, due to the unreliability of a VFR aircrafts reports.
I have also self separated in Class G whilst IFR from VFR many times and have also been subject to procedural IFR in Class C/D and also descents from Class C/D airspace into G whilst IFR.
Hope this helps, and I'm sure an ATC controller could elaborate...