So you are going to increase reliability from figures like 1 in 10^14 to 1 in 10^15 bit error rate. SAFE isn’t reading your drives like a local area network server is being read. You might get 1 chunk read every minute or hour.
A chunk is up to 10^7 bits. So in worse case a single drive is going to have a unrecoverable in > 1x10^7 chunks read, assuming the read error occurs in the chunk storage and not in other areas on the disk.
As a experienced engineer and sys admin I could not justify using RAID for these issues in this application. Just buy a better drive.
To put it in perspective, at one second per chunk read, which is considerably faster than expected average chunk read rate, it will be over 2,700 hours between read errors which is like 3 per year maximum. Your vault is going to be churned well before that. And 1 error every 4 months isn’t going to affect any reliability measure.
So yes RAID is unnecessary for vaults.
EDIT: By using that extra drive for another vault you will increase chunks being read and thus earning. This would far outweigh any benefits (if any could be found) RAID would have on earnings.