Having now spent the better part of the afternoon trying to get VistaPro to run on my system, I finally know why. The program throws up no errors, no errors, no shell output, nothing in SnoopDos. Nothing. The answer? My Workbench is set to an RTG/CGX screenmode. If I change it over to an Amiga native mode first, VistaPro starts right up, no problem. Not sure if this workaround was generally known previously, but I've spent years being unable to run VistaPro and finally decided to give it another try today. Success!
To make things a little easier, I put together a little script so that I don't need to go into the ScreenMode prefs utility every time I want to run the program. First, I saved a basic NTSC:High-Res Laced screenmode as a preset in my VistaPro directory (vistapro_screenmode.prefs).
ScreenMode vistapro_screenmode.prefs USE
Wait 5 ;you may need to adjust this higher depending on how quickly your WB resets
Stack 200000
VistaPro3.0
ScreenMode ENVARC:Sys/screenmode.prefs USE
Save the script in the VistaPro directory, slap an icon on it, set the default tool to IconX, and you're done.
The caveat is that by changing the screenmode, there needs to be a Workbench reset, so if you have any windows open that delay the reset, VistaPro will probably still fail to start. Could probably overcome this problem by replacing the Wait command with a looped command that checks the screenmode ID and breaks out of it when it returns a specific ID (i.e., matching the one in vistapro_screenmode.prefs). There's got to be a shell utility somewhere that does that.
Still, hopefully this saves some others some sanity, and if anyone knows a more elegant solution, please chime in!