That's possible. If I understand it correctly, the OTF extension indicates the font has Type1 routines attached. With a TTF extension, the font could include Open Type features and would show with an O icon). If it has a TT icon, it doesn't have the Open Type enhancements. As far as basic Windows is concerned you may not notice any difference.
All my basic Arial and Times New Roman fonts show as .ttf extensions yet show the O icon. They are open type formats.
I've not heard of anything different about Vista fonts at this time.