Having prepared an empty volume, you can take a snapshot of the application volume as shown in the following command:
# vxsnap -g mydg make source=stmrvol/snapvol=mysnpvol/syncing=on
The following command starts a full synchronization of the snapshot volume, and blocks until this is complete:
# vxsnap -g mydg syncwait mysnpvol
See Creating instant snapshots.