hi, this is a known problem.
When OpenNebula import any wild a reconfigure operation takes place, this operation writes in the vmx configuration certain aspects like the vnc port.
The problem comes when you use a machine "touched" by opennebula. for example if you try to clone the machine (vCenter side) you will be cloning openNebula configuration too, then if you try to import your cloned machine in OpenNebula you could experience a lot of problems due to the old openNebula config.
To avoid this situation before the import operation you can browse in the proper vmx of the machine and delete the config:
remotedisplay.vnc.enabled = "TRUE"
remotedisplay.vnc.port = "X"
remotedisplay.vnc.ip = "0.0.0.0"
You should delete too any opennebula reference but these are the 3 attributes that are causing the HOLD behaviour.