As others have mentioned, vDS and vSAN are not affected when you patch your vCenter. They will continue to work even if the vCenter has failed. Only the management functionalities are limited in this case.
However, since you are using VxRails with an external vCenter, the patch management process may be slightly different. I assume you have a VxRail Manager appliance and a VxRail web frontend with automatic upgrade process.
In this case the upgrade must be done in the following steps:
- Update your external vCenter via VAMI (but please check the release notes of the VxRail software what vCenter build numbers are supported for the current VxRail release).
- Update your complete VxRail cluster via the update process in the VxRail Manager web frontend (this includes all server firmware updates, ESXi version, vSAN and vDS).