We actually built a wiki version of a vendors directory! But it was too hard to use and so we will have to rebuild it as a more traditional vendor review system along the lines of what you describe above. :-)
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I am surprised that the wiki doesn't already have location pages to correspond to the location boards, but I think they would make a great addition to the site and instead of searching for a vendor with the search feature and sifting through results, we could just go to the location wiki page that the vendor is based out of and look for them as well as see other vendors there.
My idea of how this would work is each location page would have various categories (florists, makeup, venues, etc) and under each category there would be a list of corresponding vendors in the area. We could give the vendors a ranking on quality (depicted by stars or something) and also give them a ranking on price (inexpensive, moderate or expensive). From there, if you clicked on the vendor name perhaps it could take the user to a page of user reviews about that vendor.
I think the best part about this suggestion is that most of the functionality is already built in through the site, and because the users can enter info into the wikis, there wouldn't be tons of development work needed.