I will not address the strategy part of your question, because those can be wildly varied depending on your environment. The reports are not deleted if you opt to pipe them to a directory post eject, whether deferred or immediate. The \vaultname\SIDxxx folders that actually contain the original reports, as well as other data, are kept for thirty days by default.
The dupe question, since you have vault you obviously have the option to do inline tape copy. If you have tape devices available at the time the policy that contains the clients you want to duplicate runs, then I would configure the Multiple copies to different pools.
Simplistic example:
Copy 1 to Policyname-onsite-pool
Copy 2 to Policyname-vault-pool
Then in your vault profile add Policyname-vault-pool under the eject tab. This will save that overhead of the vaultrun performing to bpdupe.
Again, it all depends on where you have time available and what tape resources are available.