Vendors can only interact with document folders that you give them access to. To give vendors access to a folder, an admin needs to do the following:
Log into Client Portal.
Go to Admin > Documents.
In-line with a folder, click the orange pencil button on the right. This opens the Edit Folder window.
Click the Vendors checkbox. (Make sure the box is checked.)
Click Submit. Now vendors have access to the documents in this folder.