Yes, PostRFP provides "Project Notes" - a secure and audited message board for communication with vendors and buyers in an RFP.
"Public" project notes are messages visible to all respondents.
"Private" project notes are only visible to users within your own organisation.
Both vendors and buyers can post Notes, but Notes published by vendors are never visible to other vendors. The normal workflow is for a vendor to submit a Note asking a question - perhaps a clarification for a specific question. Then the buyer will answer the Note, publishing his response to all vendors, thus ensuring that all vendors receive the same information.
Project Notes are backed up by email notifications.