I have also tried without the umount script, but it seems that it doesn't umount automatically. Here is the log:
host:/etc/vz/conf# vzctl start 101
Starting VE ...
VE is mounted
Adding IP address(es): 192.168.0.101
Setting CPU units: 1000
Configure meminfo: 65536
File resolv.conf was modified
VE start in progress...
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host:/etc/vz/conf# mount -l | grep test
/test on /home/vz/root/101/mnt type none (rw,bind)
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host:/etc/vz/conf# vzctl stop 101
Stopping VE ...
VE was stopped
VE is unmounted
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host:/etc/vz/conf# mount -l | grep test
/test on /home/vz/root/101/mnt type none (rw,bind)
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host:/etc/vz/conf# umount /home/vz/root/101/mnt
umount: /test: not mounted
umount: /home/vz/root/101/mnt: not found
umount: /test: not mounted
umount: /home/vz/root/101/mnt: not found