If your Q&A period is active you can send messages to Vendors through Bonfire, even if they have not interacted with the project.
You'll need to start by inviting the vendors to the project.
Once you've done so, navigate to the Messages Tab and click "Start new Opportunity Q&A" (again, the Opportunity Q&A period must still be open).
When you click on the Recipients box you will see a drop-down list of all Vendors on the project even if they've only been invited.
Alternatively, you can click "All Vendors" and all of the Vendor email addresses that have been invited will populate along with Vendors who have interacted with the project.
Once you have composed your message, click to send. None of the vendors will be able to see other vendors who are recipients of your message, similar to a BCC.