I agree with tkaz. Those Billboard Magazines have great articles and the ads for the machines from back in the day. I'll take a truckload of those Popperette's at $149.50!
Also, I'll take a truckload of the used Wurly 1015's selling for $100 or less!
As far as popular coolers and machines, I think you should include the vast array of Westinghouse self serve coolers. These were all over the place and are still used today in some antiques malls and old general stores. The damn things are virtually indestructable except for rusty lids from time to time. While they were on the honor system, they were good vending displays where the customer could reach in and grab an ice-cold Coca Cola. I would have to think that the Westy's were the predominant "vending machine" in the 30's.
Another interesting angle would be the fact that Vendo built machines primarily for Coca Cola while Vendorlator supplied Pepsi and other brands.
One last interesting observation is the fact that you would never see non-Coca Cola products in a Coca Cola vending machine while you might see other brands in a Pepsi machine. That pretty much holds true to this day I believe. You also could explore the bottle/can vending machine versus the cup vendors and their different applications. Many angles that you can explore.
I would be happy to help proof your paper as you get into it. I wish I had gotten into this hobby when I was in college. But back then, I was more fascinated with the brewing industry for some reason? I actually did two in-depth term papers (25 plus single-spaced pages typed on a typewriter) on the American brewing industry for my Economics classes and used that information to make a presentation about the brewing industry in one of my Spanish classes!
Good luck!
Tim