Please note the OpenVZ recent security updates.
Kernel RHEL6 042stab094.7 (moved to stable)
Security fix preventing access to the host filesystem from inside a container. Fixes in CPT, net, iptables autoloading, etc. Stability fixes.
Now back to the vz issue. It would be worth trying to rename the current /vz partition to /vz_old and mount the disk partition /dev/sda3 on /vz. Now, you can rsync the contents from /vz_old to /vz. This way, you dont have to change anything in the confirmation, yet you will have all the VPS container on other disk.