Draft definitely makes a difference, but man do you lose resolution quality. I did change the frame rate in project properties to 23.976 and that seems to help.
Vegas is processing much less information in draft quality so your cpu is able to keep up better, hence the smoother playback. Don't forget to change your project framerate back
I always preview in draft (auto sizing) ... ONCE in a while I'll throw it up to best if I want to see up close & personal how a FX is looking, but that's more for frame by frame, as I don't expect my CPU to keep up accurately otherwise.