which is then put in a vault for a longer period of time.
The answer to your question really depends on what you mean by "longer period of time".
What if Bitcoin-Qt software doesn't exist any more when you are ready to access the bitcoins? What if computers no longer have USB ports?
A paper wallet is much safer. You can always read paper, and unless the core ECDSA protocol changes, the private key that would be printed on the paper would always be usable to spend your bitcoins.