I don't know if it helps, as none of these seems like a donut vendor, but here's the list of restaurants present at the 2012 Brew at The Zoo. If one rings a bell, you could call them and ask if they were the one with the donuts.
Restaurants
Bill’s Famous Cheese Spreads
Buckhead Mountain Grill
Chili’s Grill & Bar
Incredible Dave’s
Kern’s Kitchen/Derby Pie
Maker’s Mark Bourbon House and Lounge
Mark’s Feed Store
Puccini’s Smiling Teeth Pizza & Pasta
RiRa Irish Pub
Rocky’s Sub Pub
Roosters
Safari Culinary Company
The Irish Rover
The Monkey Wrench
Valu Market Mid-City Mall
White Castle
Whole Foods Market
ZA’s Pizza Pub