I have High Sierra and Sierra in a dual boot on 2 different partitions. Can I use a VMWare product on the High Sierra OS to run the Sierra OS as a virtual system or do I have to install a "virtual" system from within VMWare?
You could create a raw disk that points to the physical partition, as described here (requires the Terminal in the host):
Then replace the VMDK - in the ordinary virtual machine you previously created - with the raw disk components: this should give a Boot Camp-like setup.
A possible problem that can arise is that the device name of the partition can change after reboots, so it is not guaranteed to work consistently; and it's also possible that it doesn't work at all with APFS disks/partitions.
BTW, we talked about this a long time ago, here on the forum, so there should also be some old posts you can search for...