I've never used one, but here are a few things I would try.

1. Switch from serout to debug. Debug works 100% with a 4MHz osc at 9600bps.
2. Add a carriage return $0D to the end of each command string.
3. Issue the IPA command to set ASCII mode before sending OPW Test1.
4. Switch to TRUE mode if INVERTED isn't responding.

Have you read the Vinculum Firmware User Manual Version: 2.3? The VDIP1 manual looks
pretty limp, but the firmware manual looks like is has everything you need.