You will find vendor reviews in the Marketplace section of the forum.
Just click the classified add you are interested in and here you can view the specific vendors itrader or feedback rating.
Here you can see this particular vendor's rating is 6.
Once you click the feedback score you be brought to the member profile and feedback page. Here you can see all comments people have left regarding feedback of specific items they have purchased. You can see feedback from buyers, sellers, traders and others. You can contact the member here as well.
You can also view a members vendor feedback by clicking on the members name anywhere on the forum. This will bring you to the members profile page. You can find out a lot about a member here, but we will stick to vendor feedback. Click "View full profile for ...."
This will bring you to the vendor feedback page, it is just a different way of getting there. Once there and you would like to leave feedback take the following steps.
Click "Submit feedback for ...."
Choose if you were the buyer, seller or trader.
Choose your overall experience: Positive, Neutral or Negative
Write a short comment about your experience that will be shown to others and also add a link to the add you used to purchase the items in the Deal thread URL space. You can also choose to write additional comments to the vendor that only they will see.
Once you hit submit you be brought to the vendors feedback page and you will be able to now see your feedback that you have left.