I'm having a similar problem after upgrading to 11.1-RC3 today.

I was able to find the following log entries when I try to boot up a VM that I was not able to connect to with VNC (lines are wrapped manually for clarity):

[2017/12/05 02:24:30] (DEBUG) VMService.run():153 - ====> NIC_ATTACH: igb0 [2017/12/05 02:24:30] (DEBUG) VMService.run():229 - Starting bhyve: bhyve -H -w -c 12 -m 4096 -s 0:0,hostbridge -s 31,lpc -l com1,/dev/nmdm2A -l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI.fd -s 3,virtio-net,tap1,mac=fe:ed:00:42:69:d2 -s 29,fbuf,vncserver,tcp=172.16.69.161:5902,w=1280,h=1024,,wait -s 30,xhci,tablet -s 4,virtio-blk,/dev/zvol/datapool/vm/archvm/os -s 5,ahci-cd,/mnt/datapool/share/ISOs/archlinux-2017.09.01-x86_64.iso 2_archvm [2017/12/05 02:24:30] (DEBUG) VMService.run():248 - archvm: 04/12/2017 18:24:30 ListenOnTCPPort: Can't assign requested address

Then, when trying to debug this I looked at at netstat, and there was nothing listening on that port, so I'm not sure I understand what would be causing this "Can't assign requested address error".

Let me know if there's anywhere else I can look for more details.