Allowing for unbiased vendor reviews by users is probably smart, but I wouldn't restrict vendors or prostaffers from posting elsewhere. That seems a little too censored. Just moderate as needed in other subforums. A "brand specific" subforum where vendors can interact with users and post promotions and whatnot wouldn't be terrible either.
Also just another idea: Maybe require and/or supply vendors and prostaff with a badge so that users can see them and there is a little more transparency. Not sure this would carry over to tapatalk but at least on the website it would have visibility.
So, for example: DaveT could post in the 'Brand Specific' subforum and general forums to his hearts content and discuss the phantom, nucanoe, and crocs to his hearts desire, but since he's 'prostaff' he would not have permission to post in the 'vendor reviews and information' subforum. If he, for some reason, starts spamming 3.7% off discount codes in the general forums, those posts could be moderated by the mods, vice versa for slamming other vendors, their products, users, etc.
Also, have you guys thought about a general 'code of conduct' for vendors/prostaff, heck even users at large? Might be good to set the rules up front, or in a sticky post somewhere.