As I understand you have 2 network interfaces (eth0 and eth1) on host node, which are connected to different network segments (outside network and virtual subnetwork 192.168.156.0). And you need to configure 2 veth devices to be able to reach these network segments.
Besides steps from http://wiki.openvz.org/Virtual_Ethernet_device you need to add routes inside VPS:
[vps]# ip ro add 192.168.156.0/24 dev eth1
[vps]# ip ro add default dev eth0
Also you will need to add NAT rule to iptables on host node:
[host-node]# iptables -t nat -A POSTROUTING -s 192.168.156.0/24 --out-interface eth0 -j SNAT --to-source 200.25.123.2