If you have the Opportunity Q&A feature enabled and the Q&A period has not expired yet, you can send messages to vendors through Bonfire, even if they have not registered or otherwise interacted with your project.
First, invite the vendors to your project. Once you've done so, navigate to the Messages section on the Project Details page and click Start new Opportunity Q&A (again, this button will only be visible if the Opportunity Q&A period is still be open).
When you click on the Recipients field you will see a dropdown list of all Vendors associated with the project (including those that were invited and have done nothing else).
Alternatively, you can click the All Vendors button 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 Send. Note that the recipient list acts similarly to a BCC email, and none of your vendors will be able to see the other recipients of your message.