Arc250 will take it apart on an emulator or the real thing..
took me a while to find arc250 and take a look at the files,
it's not the last version I released, so it's missing the $FFD2 hook ( and a cheezy simple paint program that works in that I-Paint/basic-8 interlace mode..)
but the prg's are there to init the VDC properly and copy over the charset etc.. I'll dig around this weekend and see if I can find the floppy where I have the $FFD2 hook , and all the source code for the prg's.. and put them up...

Sorry about raising the dead, but in case someone was actually looking for an answer to this topic..

Years ago I designed a hardware project to connect a VDC (and 2nd sid) to a C64, I also included software to set the 80Col mode, and patch the kernal $ffd2 to display text/BASIC & editor output on the VDC, it also has an Ipaint viewer for 64mode etc..

The hardware maps the vdc in the same location as on the 128, so all the software also works on a 128 in 64 mode..  The software & Schematics are here..

http://cbmfiles.com/genie/geniefiles/C64-128Toolkit/VDC64APRO.ARC