Unless you want to download your legally bought games and stuff from PSN. But you'd need to upgrade your Vita's firmware to most recent one though.
If you have a backup of activation files from both accounts, replace the data from the account you want to have on your Vita and remove the ones from the account you want to "remove". I've done it on my Vita, but I'm using the same account on two of my Vita's and both are activated, so I'm not that sure it will work.