Ebay the Titan and get a 390x. Vegas needs a card that supports OpenCL better than NVidia anything. The 390x will significantly accelerate your Timeline, and small aspects of rendering. You still need to render the timeline during render, so you do get GPU assist during renders.
Uninstall the Nvidia drivers back to Windows default, reboot, then shutdown again, before pulling the GPU. Install the AMD as normal, card then drivers.