I spent days trying to make Vegas Pro 16 (build 307) render without success. The solution which worked for me was to disable one of the cores on the Intel I7 chip on my machine (with Vegas loaded start Task Manager, go to Details, find the Vegas 16 process and right click on it. Select "Set Affinity" and un-check one of the boxes.) This ensures that one core is not used for the rendering and the chip does not run at 100% capacity.
I then shut down all unnecessary processes. Programs running in the background which check for updates, drivers, emails and Dropbox synchronizations all worked at critical times to make Vegas freeze. The computer should be dedicated to the rendering and nothing else. It now seems to work.