It appears to be perfectly possible to pass, e.g. -s 5,virtio-net,vale0:vm1 to bhyve and it will create a virtualised network attached to a VALE switch. It should be listed in bhyve(8) alongside the tapN, vmnetN and (for FreeBSD 13) netgraph options.
In practice, I didn't get very far when trying to get it to work. Attaching my physical interface with valectl -h appeared not to work so perhaps that depends on driver support. I think I had it working for networking between VMs.