[Stampercam]

I get all my textbooks now via ebook but each publisher seems to want their own app or proprietary reader. I'd like to have all my titles in calibre and I've been able to with most of my titles, however I have a couple in .vbk format that calibre doesn't see as a book. I've put the title in, and when I try to view the book it just opens the VitalSource program. This is great if I want to read on the computer. The downside to this is that I can't put the book on my device to read. I can read it using the vitalsource app on the ipad but I'm kinda frustrated with having so many books in different apps on the ipad, and would rather use them in the reader of my choice.

So, my question now is - does anyone know how I can convert .vbk to a more useful format?