Vendor Invitations are sent immediately regardless of whether a Project is in the Pending or Open stage. Once a Vendor is invited to a Project, you could delete the invitation from the list, but based on scenarios listed below, you will have different outcomes:
On all Projects, the Invite will go out with a future date/time if it the project is still Pending, and will indicate immediate access if the Project is Open.
On Public Projects, deleting the invite will not affect the invite that was sent to the Vendor's email.
On Private Projects, deleting the invite will revoke a users ability to access your Project.