Studio 12 has been a step backwards regarding Importing DVDs, particularly those recorded by cameras or standalone recorders. Have you tried a regular (unprotected) DVD or one made in Studio? If that does work, then there are a number of fixes available.
Upgrading to 12.1 includes a fix for some DVDs. In the knowledge base, there is a further patch and discussion HERE
However, for me, that hasn't fixed the issue for some of my "problem" discs. I have reverted to the S11 DVDreader.DLL by putting that into the 12 program folder and solved the problem for now.
Some more info HERE and I think the 11 dll is posted somewhere, I'll get back.....
ADDED - another thread HERE
and S11.1 dvdreader.dll is HERE
BTW, all my testing has been with XP, so the final fix might not work in Vista (or at all on some discs!)