Hi Fadi,
Within Vegas, here are some thoughts you may or may not have tried……
Reduce the preview window quality.
Increase the Dynamic Ram Preview from the default of 200, to maybe 5,000, then mark an area in the track you want to preview smoother, and then ‘Build Dynamic RAM Preview’ it is ‘Shift+B’. That probably will not perform much better, but may, it’s worth a try.
Similar to above except ‘Ctrl+M’ will render to a New Track and be inserted into a new track above. Mute the original track and it should then run smoother in those areas. Select a render format and template that will provide a descent preview quality, but run smoother and take the least time to render……you find the best format and template and custom settings with a bit of trial and error.
Render your project out to an intermediate something like Cineform (.avi), or Avid DNxHD (.mov) and view it in a 2nd instance of Vegas Pro or an added track in your current project. I of course don’t know how complex your project is, but these are just a few thoughts.
Best Regards……George