It simply goes along with previewing and will happen in any software. There is too much to be done for the processor to generate the frames fast enough for smooth previewing. This is why rendering usually takes longer than the video lasts. When rendering Vegas will take as much time as it needs to generate every frame perfectly. It doesn't have time to do this while previewing so it just shows you what it can and skips the frames it can't do.

If you want to see a transition smoothly in preview, highlight it as a loop region and press Shift-B to generate a RAM prerender. It may take a few moments, but then that section will play smoothly at full speed ... until you make other changes.