Will deleting a Vendor invite prevent them from accessing the project?

If you remove a Vendor who has not yet submitted on an Invite-Only project, they will not have access to the project from the moment you've removed them. 

NOTE: The Vendor you are uninviting will have already received the initial project invite to their email inbox. They will therefore be aware that the project exists.

If the Vendor also downloaded documents from your project, and you send a Public Notice, they will also still be able to still see that public notification.