Don't use PF24, if anything you are losing quality in your image by having to do inverse telecine to get 24 progressive frames out of 60 (which doesn't even divide evenly), so on paper that mode looks to be completely worthless. Buy a camera that can do actual 24p like one of the Panasonic GH series cameras. Edit: Just saw where you said it was accidental.
That said, sometimes Vegas' resampling can create that ghosting effect, see if disabling it in the project settings has any effect. If that doesn't help, I don't know what to tell you.