I know this is going back a ways, but if someone else has any questions about this build, I just put one together for my VT20+
I modified a few things to keep stock standard parts and all seems to work ok. I ran mainly off the schematic from here:
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/VOX/Vox_VSF5_footswitch.jpgAnd the vero build I put together (WARNING, these are the parts that I used and works for the VT20+):