How do I PROPERLY slow down a 60 fps clip in Vegas Pro 11?
You should be safe setting playback rate at 0.400. 59.94 is actually 60/1.001 and 23.976 is actually 24/1.001, so the 0.4 multiplier is technically exact. Switch off resample to be sure. Of course, how Vegas actually handles this internally is anybody's guess.

One way to test this would be to generate a frame sequence just containing an incremental numeral display (1,2,3...) in AE or similar, then render that as a 59.94p video. Import in vegas with playback 0.4 and resample off and then cycle through the frames to see if there are any duplicates or missing.