Hi oldbob and welcome to the forum! I'm sorry for not noticing your post at first, but I'll be glad to help in any way I can since I'm one of the few other Matrox TH2Go users here (and probably the only one that works for them too, lol). Your issue
may be related to how your Triplehead2Go is being detected in Vista. Please try following the steps in this link:
Matrox Graphics - Support - TripleHead2Go Digital Edition - Widescreen Resolutions (Troubleshooting) in order to replace the .inf file so that the TH2Go is properly detected instead of being believed to be a "Default monitor". Let me know if/how well it works!