You can use /etc/vz/conf/vps.mount & /etc/vz/conf/vps.umount scripts, here is how i've done it:
projsrv3 ~ # cat /etc/vz/conf/vps.mount
#!/bin/bash
if [ -x /vz/root/${VEID}/.cs-chroot.start ]; then
/vz/root/${VEID}/.cs-chroot.start
exit $?
fi
projsrv3 ~ # cat /etc/vz/conf/vps.umount
#!/bin/bash
if [ -x /vz/root/${VEID}/.cs-chroot.stop ]; then
/vz/root/${VEID}/.cs-chroot.stop
exit $?
fi
projsrv3 ~ # cat /vz/root/103/.cs-chroot.start
#!/bin/bash
mount -n --bind /vz/template/portage $(dirname $0)/usr/portage || exit ${?}
mount -n --bind /vz/template/portage-distfiles $(dirname $0)/usr/portage/distfiles || exit ${?}
mount -n --bind /vz/template/portage-packages $(dirname $0)/usr/portage/packages || exit ${?}
projsrv3 ~ # cat /vz/root/103/.cs-chroot.stop
#!/bin/bash
umount $(dirname $0)/usr/portage/packages
umount $(dirname $0)/usr/portage/distfiles
umount $(dirname $0)/usr/portage
projsrv3 ~ #