Imported VMs in vCenter do not support VNC, we have updated the documentation to state the limitation, and opened a ticket to keep track of the solution:
I suppose another issue regarding VNC will be to not force you to delete all VM's from opennebula and import again for enable VNC, if VM's was imported into opennbula from vCenter without VNC.
Imported VM’s are assigned to User (delete VM and import suppose to resign VM to the user)