MacBookPro8,2 (early 2011) RAM size limit per Bank as the titles states I'm curious about the maximum supported RAM per Bank on a MacBookPro8,2. 

I know the total supported capacity is 16GB but can this be achieved with just one DIMM leaving the other bank empty?

I'm asking this specific because recently one of my RAM banks died cutting my
available RAM in half.
(Luckily it's not the complete logic board, I just run out of warranty..)
If the limit is only in the total size and not a per-Bank limit then I would just get new DIMM to put in the remaining functional Bank.

Also which - if any - vendor would you recommend for DIMMs this size?
Apple states that you should fill both RAM banks with equal size memory for best performance, implying that one bank should work.  Your machine running on one bank would seem to bear this out.

For the second part of your question I can recommend RamJet (1-800-831-4569) as both a good source of quality technical support that is Mac specific, and a retailer of RAM.

I have no business connection to them, but I have been using them for my own machines and my clients machines for almost ten years.