The choice of vif-plugging for the NEC plugin depends on what version of libvirt you are using, as well as whether you are using Nova security filtering (i.e., security groups, provider firewall, VM spoofing prevention).
When using libvirt (any version) with Nova security filtering: libvirt_vif_driver=nova.virt.libvirt.vif.LibvirtHybirdOVSBridgeDriver
When using libvirt (version < 0.9.11) without Nova security filtering: libvirt_vif_driver=nova.virt.libvirt.vif.LibvirtOpenVswitchDriver
When using libvirt (version >= 0.9.11) without Nova security groups: libvirt_vif_driver=nova.virt.libvirt.vif.LibvirtOpenVswitchVirtualPortDriver