ANSWER
Hey guys, this is the best way to get rid of choppy preview/render everything. I had exactly that problem, and it really made it mad.
Load up sony vegas go to options>preferences>video>GPU acceleration of video processing>off
And close vegas, open it again and it works!!!
There you go! Anyways that’s the ONLY way I found it works. If it did please subscribe to my youtube channel, its name is lolOptixHD and like my videos!!
Thanks 🙂