I have a 23Cii, and an older Aristo Cold Light head that fits into the former filter slot of the condenser head. So, where would you place VC filters above the negative? I know I can go below the lens.

Les,
You can fabricate a filter slot to fit between the lamphouse and neg stage.
I made one from plans derived from an article in C&D years ago.
Easiest way to describe it would be as the letter "U" sandwiched between two pieces of masonite with a large hole cut through it.
Think of it as a thin, three sided drawer.

I had to deal with this exact issue some years back. I had the older Aristo unit and bulb (#51?) that needed some correction to make it behave better with VC filters. I can't recall the correction filters as I write this but I had them taped over the bottom of the lamp housing. These were some old cc filters from a Beseler color printing filter kit. I was using VC filters simply placed over the negative carrier when sliding it into place. Not very elegant but it seemed to work. I later changed bulb/tube to the newer version but eventually went to a dichro unit and I prefer that after using it in condenser, then cold light, then dichro modes.

just place it on top of the negative carrier. Works fine, however, if you are split printing that is not a good option as one tends to move the negative a bit which usually creates focus issues on the easel.