If you want slow motion to happen automatically you need to shoot in one those overcranked 24p modes. CineForm products detect the frame that you wanted to edit in (as set by the camera.) So you current file must be marked as 60p not 24p -- so you didn't shoot for slow-motion. That doesn't matter because you correct that in post. When you put 60p files on a 24p timecode most NLEs correctly match the orginal play length -- so Vegas is doing the right thing (skipping or blending frame to keep the orginal play speed.) You need to tell Vegas to interpret the footage as 24p.
thanks, both of you. Just for reference, another way to do this without having to work it in post, is to shoot 24Pn as opposed to 24p. N stands for native...