NICs present in the vCenter VM Template won’t be honored by OpenNebula:
From the documentation:
vCenter VM Templates can define their own NICs, which OpenNebula cannot manage. However, any NIC added in the OpenNebula VM Template, or through the attach_nic operation, will be handled by OpenNebula, and as such it is subject to be detached and its informatin (IP, MAC, etc) is known by OpenNebula.