I have invited vendors to a Project that hasn't opened yet, will they receive a reminder once my Project opens?

Email invites for Projects only send once the project is in its open stage. Therefore, if you have sent an invitation to a Vendor while the project is pending, the email notification will send to the Vendor once the project reaches the open date:

If at any time you need to manually resend the invitation once the project is open, you can navigate to Vendor Invites on the left hand side, and then select the Invite Vendors tab. From here, open the Actions drop down menu and select Resend: