First project was crashing on render with build 636 so I went back to 550 and had to do the project over. Hoping it was just a freak occurrence, I updated to 636 again and tried to render a second project and 75% of the way through the render, Vegas just disappeared! It apparently doesn't like my Nivdia GTX 1070 graphics card.
I had a successful render after updating the Nvidia graphics driver to version 472.84; hopefully that's the answer.
I upgraded to driver version 512.96 per post below...