There are many advantages to using ploop rather than simfs. The advantage of course I choose is file security. While simfs places all the files directly on the hostname for ease of use and managing, ploop creates a virtual disk file and then put files inside this disk. When the virtual machine is started vzctl has to mount the ploop disk. Versus with simfs, there is no mount involved.
vzctl convert [--layout ploop[:mode]]
vzctl convert --layout ploop <CT_ID>
MAKE A BACKUP FIRST