When having users upload videos to my vimeo via a form in a WordPress blog, can vimeo return the video URL (to use in the code) before the video upload is actually finished?
No, this is not possible.
The resources required to define an upload url are all created during the completion step of an upload (in POST uploads this is the first redirect to vimeo.com, in PUT uploads this is the DELETE request).
This allows us to verify that you have uploaded a video (and not a text file, or image), and verify that the upload has completed, and is successful, before assigning a URL.