Currently in 3.2 when we refresh a VDB we do retain the old timeflow, (the old snapshots of that VDB, etc. that is being refreshed) as part of a separate timeflow, however they are still managed by the retention policy that you have set and will be deleted according to that policy.
Additionally this timeflow is not displayed in our GUI. However you can see this via CLI.
To avoid the retention policy removing something that could be needed by you: 1) Create a snapshot of the VDB from the point in time where they want the back up from. 2) Then create a new VDB from that snapshot and call the VDB <VDB_NAME>_backup. 3) After this <VDB_NAME>_backup VDB is created verifying you have a good snapshot and then you can, stop or disable it. You can then use the capacity UI to mark any snapshot as keep forever, just in case. 4) When not need any longer just delete this <VDB_NAME>_backup.
Here is our 3.2 doc link that discuss refreshes: http://docs.delphix.com/display/DOCS32/Refreshing+a+VDB
Our 4.O documentation has more information around this, here is our 4.0 doc link: http://docs.delphix.com/display/DOCS40/Refreshing+a+VDB
And our 4.0 doc link for the CLI around rollback and forward: http://docs.delphix.com/display/DOCS40/CLI+Cookbook%3A+Rolling+Forward+a+VDB