You imagine well that I won't test to find or not if a debian really needs efi vars. Fact is that all EFI VM MUST have its efi vars like a bare bone PC has. Nevertheless, I can look if debian now writes a /boot/bootx64.efi (see below).
Several people have reported that a debian bhyve VM doesn't work at reboot. And it's because debian set its own loader path/name in the ESP. It's not the default /boot/bootx64.efi. It then uses efibootmgr to set its own loader name as an efi boot var. But if you don't have an efi vars file, the VM forget it at reboot. It's as simple as that.
bhyve didn't support efi vars at some point in the past. So, copy the debian loader to /boot/bootx64.efi was mandatory. It's not anymore the case since some years.
FreeBSD does the same as debian but keeps writing /boot/bootx64.efi (if not already exists) in addition to its dedicated loader.
Concerning the Op problem, the fact that vnc can't connect shows that the VM isn't running (or bhyve has crashed). It's likely a vm-bhyve problem. It has some logs, you should look at them.