This one is semi-verified: I have built it on my custom protoboard but I am posting a layout adapted to a more normal breadboard (mine has 3 rails on the edges and a central ground rail). Pass up on this one if you don’t feel capable of cross-referencing the schematic as you build.
This is the Skull & Circuits Vactrol VCF-1, probably as simple as a usable VCF circuit gets.
I used a pair of 1N4148 for the filter diodes instead of LEDs - actually I added both to my board so I can rewire it later. Both sound different. The LEDs do not light up when used as part of the filter.
The LED I put on my panel connected to the audio output with a signal diode antiparallel instead.
Also I used a single TL074 instead of two 72’s. Ok actually I used a TL084 if you wanna be all technical about it. Works all the same.
Edit: The TL074 is upside-down!! Sorry I didn’t make it more obvious in this version of the picture. Pin 1 is in the top-right.