TLDR: It is not actually supported.
Full version: If you build a Big Sur VM on vSphere 7, under virtual hardware 18 or 19 on legitimate and officially supported Apple hardware (I am running mostly Mac Pro 6,1 with a few Mac Mini 8,1 - both on the supported list under 7.0.2), it will not be able to update unless you put a hack into the vmx file. I am not going to say what it is to avoid my post being nuked, but I discovered this when I searched for the issue and found that people who are breaking the macOS license agreement to Mac macOS Big Sur run in VMware Workstation had a workaround. Then I tested their workaround and it works if you use it in vSphere 7. I filed a support request with VMware and during a Zoom session with me they observed and then verified the issue. So did they go ahead and make it a bug report so that it would get fixed? NOPE.
Instead... VMWare support waited a day and then... suggested I try downgrading to virtual hardware 18 or 17 (which makes no difference and 17 is not officially supported for Big Sur) and simultaneously said they don't think it is their issue and suggested that I should ask Apple to remove the additional hardware validation step that Big Sur uses. This is only reasonable if you also believe that if Windows 10 refused to update when running in VMware, VMware would tell me to call Microsoft to change Windows.
Would implementing an official version of the hack be trivial? YES (they literally need to pass through exactly one variable from the host hardware). Does official support for Big Sur exist in vSphere 7? OBVIOUSLY NOT.
My recommendation would be to take your SR number and talk to your TAM (if applicable) or your account manager and get some visibility. This doesn't sound like a "proper" response from GSS and sounds like an enhancement is needed in the product. Shopping that SR around internally is a good way to publicize it for resolution.
Here is what actually happened. they stated flat out that they have no ability to access an environment in which macOS guests are supported. So support literally CANNOT support anything that involves a macOS guest. They are selling a support contract with exactly zero ability to provide support. After verifying the bug, and after verifying that it is fixed in the VMWare Fusion/Pro virtual hardware, support demanded that I provide logs from vCenter before they would proceed, as if that was in any way relevant or necessary. It's a tick the box exercise that just papers over their inability to test or replicate in house for supposedly supported configurations.
Months later this is still broken and we are still using a hack to be able to patch guests.