In any case in OpenNebula, if the VNC icon is disabled while the VM is in the running state it would be caused by the absence of a GRAPHICS/TYPE attribute element set to VNC. You could check if the VM has that attribute in the template tab for the VM (using the admin vcenter view) or running a onevm show VMID -x CLI command (replacing VMID with the ID of the VM).
Let’s see if that GRAPHICS section is there and then let’s see why it’s not there because if you have a VM running in vcenter, you poweroff the VM, add the remotedisplay.* and then import a wild tab that GRAPHICS section should be there. But it would be better to use the vonecloud forum instead.