At a trade show, I bought a utility that allowed me to run PC-DOS under PC/IX. I'm sure it wasn't a virtual machine. Rather, it just swapped back and forth. (Guessing a bit there.)
Hmm ... you sure it was not either VPIX or DOS/Merge -- ISC built VPIX in cooperation with the Phoenix Tech folks for PC/IX. I always bought a copy with it, but it may have been an option. LCC did DOS/Merge originally as part of the AIX work for IBM and would become a core part of OS/2 Warp IIRC. Both Merge and VPIX had some rough edges but certainly worked fine for DOS 3.3 programs. The issue tended to be Win and DOS graphics-based programs/games that played fast and loose, bypassing the DOS OS interface and accessing the HW directly. For instance, I never got the flight simulator (Air War over Germany) for Dad's WWII plane (P-47 Thunderbolt) to run under either (i.e., only under DOS directly on the HW. FWIW: In that mode, Dad said the simulator flew a lot like how he remembered it).
Both Merge and VPIX used the 386 VM support and a bunch of work in the core OS. Heinz would have to fill us in here. The version of the 386 port ISC delivered to AT&T and Intel only had the kernel changes to allow the VM support for VPIX to be linked in, but it was not there. IICR (and I'm not sure I am) is that Merge could run on PC/IX also, but you had to replace a couple of kernel modules. It certainly would work on the AT&T and Intel versions.