OK, so I was using V7.5 for all the stuff I was trying to do. I fired up the latest V8, made sure the bootloader and firmware was updated to the latest release, and read the TCM with V8. Since V8 doesn't allow any mods at this point, I opened the cal under V7, changed the VIN, and resaved. I then opened the modified .ctd file with V8 and reflashed the TCM. Afterwards, I disconnected the ignition power only and waited for a full minute (this was done on the bench). Once again, the VIN did not take :(

FWIW, I have made some changes to the defuel tables and reflashed those, and upon read back they were indeed modified. Strange that the VIN stays blank though. Anyway, I will try to get it reflashed with a Tech II sometime this week and see what happens after that.