I assume you have the DD setup as a disk stunit?  If so, use Vault to duplicate 
those backup images to real tape and it has a option for "expire original disk 
images are XX hours".

If you are using VTL, you can still use Vault, but you'll need some custom 
scripts to identify backups that have been written to VTL tapes and to then 
expire the individual backupids after they've been verified as copied to real 
tape.  (Vault does not support expiring of original backup images from tape).