The package works with vimeo, goto the vimeo api and look for:
Vimeo::uploadImage
You should be able to specify which folder.
Edit: Study over their API for a better understanding: https://developer.vimeo.com/api/reference/folders#add_video_to_project
https://developer.vimeo.com/api/reference/folders
https://developer.vimeo.com/api/upload/videos
Also the upload code is at:
https://github.com/vimeo/vimeo.php/blob/master/src/Vimeo/Vimeo.php See line 316. Then line 575 is called, so the code should give an understanding of what is happening and how to specify a folder.