GitHub:
https://github.com/GMMan/Extractor.epub
This is an EPUB file that allows you to take EPUBBook content from VitalSource Bookshelf and convert them into a regular EPUB file that you can use on the ebook reader of your choice. Simply sideload Extractor.epub into Bookshelf, insert the file name of the book you want to convert, click the Extract button, and download the result.
[
snip]
Update: As of Bookshelf 6.7.0 (16), EPUB addresses for VBKs are randomized and OPF files can't be obtained, so this will not work for that version of Bookshelf and beyond. For whatever reason, they they haven't fixed cross-book requests, which was what allowed this in the first place.