I was able to get pas this hurdle by, as referenced at the bottom of the Vagrant link from above, as well is in another local thread here by adding the nfs_udp: false, configuration to Vagrantfile.
I added it in two places, on lines 86 and 87.
I had wondered if **extra_options were command line options where this config pair could go, but it appears they are settings for when vagrant_mount_type == 'smb'.
UDB, is User Datagram Protocol. It looks like Big Sur has added some security features that may have effected these types of connections.