PlayStation Minis obtained for free through Sony’s premium subscription service, PlayStation Plus, do not work on the PS Vita. Members of the official PlayStation forum discovered the unfortunate bug and raised their concerns online. Sony's said it is "aware of the issue, and will resolve it as quickly as possible".
To clarify: PlayStation Minis purchased outside of PlayStation Plus work perfectly fine, it’s just those obtained for free through the subscription service that kick out an error. If we had to guess, we’d put the issue down to the expiration date that comes attached to all PlayStation Plus downloads.
Hopefully the problem’s fixed soon – we’re a couple of levels away from getting 100% in OMG-Z.