I've never used OPC, so I can't tell you much about how JFBConnect would work with it. I can tell you that JFBConnect creates 100% standard Joomla users that work with any extension and we already have support for VM2 for importing user profile information.
My guess/hope would be that you can use our tools for easily adding the social login buttons to the OPC page (just add {JFBCLogin} where you want them to show). JFBConnect can be configured to leave the user on the "Same Page" after they register or login. In that case, the user will get to the OPC page, login or register and be left on the same page after their account is created.
Again, we haven't tested that, but it's how I would assume it would work. If you test JFBC with OPC and run into issues, let us know what they are.
Thanks,
Alex