It turns into a "fine line" on a "slippery slope". Having run a chat board in the past, the admins are walking a tight rope with different "slants" on every thread. IE: one member who's had good luck with a particular vendor or product may have an opinion, and another member may have had a different experience, with a 180 opinion. With so many opinions, how do you level the playing field?
Every vendor has preference for the products they sell. That's what puts the food on the table... and it figures. Inviting vendors to participate in discussions can have effects on the direction a thread takes. There's pros and cons to each side of the issue. What's fair is really the question to be asked, and what's fair to me will be different than fair to you.
Here's an example: one customer has an issue with a fixture that is defective. Contacting the vendor had a poor result. Both customer and vendor are members of the same chat board. One opinion is expressed, and now feathers get ruffled. What is the administrator to do now? That's the slippery slope. I've seen the "tendency" of other members who will now "gang up" on the negative member's comments if they don't agree (right or wrong). Having vendors who are allowed to participate on any board can be a very good thing and a very bad thing. Have a vote and let the majority rule... or... let the admin make the call. After all, it's his/her board! There is no "correct answer".