Some annotation to the last sentence of this page:

if you get a warning like this: umount: /vserver/vm_base: device is busy it probably means that you accidentally started some daemon out of your chroot. At least it happened to me. I chrooted into the vserver disk, installed ssh and it immediately stopped my regulary ssh daemon and started that of the chroot instead. That was the cause of the error message.

Otherwise thanks for this document!