When a vendor submits a question using the Vendor Discussions feature found under the Messages section, the question can only be seen by the buyer and not other vendors. Likewise, the buyer's response is sent as a private message that only the asking vendor can view.
That being said, the buyer may choose to utilize the Public Q&A feature, or answer all questions in a Public Notice or Addendum that can be downloaded from the Supporting Documentation section. This makes the questions and answers viewable by all vendors and to prevent the same question from being asked repeatedly.