As someone that started one of these threads a while back about a negative vendor experience, I have a thought. Perhaps instead of having a policy like this which is going to be difficult, and subjective, to enforce, why don't you have a policy instead that prevents the "piling on" that many people do? That doesn't mean that the thread has to be locked necessarily to prevent the piling on, but as moderators you can be proactive as to stopping it.
If people cannot post their experiences for fear of being blocked of banned from the site, or even just 'called out', then I fear that we lose the ability to share those experiences to educate future shoppers.
Personally, I wish I had seen more threads about vendors in the past, for some reason I only recall seeing those for the last year, perhaps 2. Perhaps the problems only started more recently such that there wasn't as much to report before that. Whatever the reason, it seems to be a more recent phenomena.
My vote, if I'm allowed one

, would be to allow the forum users to share their experiences, but to put a policy in place where the experience need to be fact based. Then warn people (and have as part of the policy) that piling on is strictly prohibited. If someone has an experience they feel the need to share, they should start their own thread that must abide by the same policy. I usually don't feel that the initial post in these threads is so bad, but the follow on is where they tend to go downhill quickly...
Could some variation of that idea work?