Just clone the vm and upgrade it to Mojave.
ESXi 6.7 and Fusion 10.1.X both support booting APFS so you are OK.
Unfortunately, that won't work. The High Sierra VM doesn't have enough free space to upgrade to Mojave. In addition, if I were to somehow erase "unneeded" applications to make room for the Mojave installer, it would be the Beta 2, which, for now, won't work on a Mac Pro 5,1. It says it needs Metal, which the Mac Pro has, but Fusion doesn't.
EDIT: I just made the High Sierra APFS virtual disk larger. As regular Terminal instructions wouldn't let me resize the APFS container, because there was a small unused volume "after" the regular Macintosh HD volume, I resorted to Paragon Hard Disk Manager for macOS, deleted the offending volume and enlarged the Macintosh HD volume. After that, I installed Mojave Beta 1, which ended up with a non-working Finder and, finally, updated to Beta 2. Now everything is great. Thanks.