Few months back, VMWare has released VDDK version 9 https://developer.broadcom.com/sdks/vmware-virtual-disk-development-kit-vddk/latest 

Currently VDDK version 8 is mentioned/tested in latest(v2.10) version of MTV documentation

MTV documentation:  https://docs.redhat.com/en/documentation/migration_toolkit_for_virtualization/2.10/html/planning_your_migration_to_red_hat_openshift_virtualization/assembly_provider-specific-requirements-for-migration_mtv#creating-vddk-image_mtv

Upstream documentation https://libguestfs.org/nbdkit-vddk-plugin.1.html#SUPPORTED-VERSIONS-OF-VDDK says VDDK plugin should work with future versions.

Please test the VM Migration using VDDK versions 9.0.0.0 and 9.1.0.0 and confirm the supportability in MTV documentation.