One last post on this, as the problem is now solved.

As a last resort, I built a new ISX .ISM for a vfp8 app instead of using the warmed-over vfp7 setup. Eureka. Whatever was wrong is fixed now. That's what I get for being lazy and trying to use the old ISM to save work <g>

The vfp runtimes are installed in NT4's "Program Files...Shared" as they should be, and the vfp8 app runs fine.

So I can say without doubt, an ISX 3.54 vfp8 setup on NT4 works fine.