I just managed to upgrade a macOS Sierra 10.2.6 VM to High Sierra 10.3.1 on Fusion 8.5.10 on a macOS Sierra 10.2.6 host. Now the VM has also updated to High Sierra 10.3.4.
A few weeks ago, I did try the High Sierra upgrade on the Sierra VM but it failed and rolled back. The difference is that the security update over this Easter weekend of the Sierra 10.2.6 host seems to have included the Spectre microcode update. I have a 2014 MacBook Pro with Crystal Well i7-4770HQ CPU. Now the Spectre microcode updates are also exposed to the macOS VM. Maybe that is the reason why the upgrade to High Sierra succeeded this time.
But I don't see the Spectre microcode updates mentioned in the Apple security bulletin https://support.apple.com/en-us/HT208692
You can check the vmware.log of any VM.
The EDX register value of 0x0c0000000 in CPUID leaf 7 indicates the Spectre microcode is present. It will be 0x00000000 if the Spectre microcode update is not there.
| vmx| I125: hostCPUID level 00000007, 0: 0x00000000 0x000027ab 0x00000000 0x0c000000
The 3 Spectre related microcode should be found by Fusion 8.5.10
| vmx| I125: Capability Found: cpuid.STIBP = 0x1
| vmx| I125: Capability Found: cpuid.IBPB = 0x1
| vmx| I125: Capability Found: cpuid.IBRS = 0x1
The screen refresh is not so slow on the High Sierra VM compared to the near slow-motion refresh of the Sierra VM.
