Just wanted to add a little more info..
Two things:
– Twixtor can do a better job at velocity changes than Sony Vegas. When you say there are “double moving elements” that is the result of frame blending that Vegas does. So in this case Twixtor will probably do a better job and improve the quality of the slow-mo footage.
– A slower shutter speed in the camera will produce more motion blur because the shutter is open longer. You were wondering if you will get better results if there is more motion blur in the original clip when you retime it in Sony Vegas because it will smooth the results of the frame blended results Sony Vegas gives. Personally I wouldn’t do that because then everything will be soft even when not retiming with Vegas. You should try Twixtor.