Via 'man vzctl', for veth, you should end up with something like:
vzctl set <VMID> --netif_add eth<N>,,,,vmbr<M> --save
for adding an interface eth<N> in the container <VMID> to bridge vmbr<M> in the host.
To remove eth<N> from container <VMID> trigger:
vzctl set <VMID> --netif_del...