VectorMotionBlur causing hang on render

I have a timeline with normal footage and a fusion composition. When using normal "motion blur" via Render3d node, I can export/deliver the file (very very slow 0-.5 fps).
When I disable motion blur and use a separate VectorMotionBlur node, the results are MUCH better and MUCH faster when in Fusion, BUT when I goto deliver, the second it hits the Fusion comp, render fps drops to 0 and the whole program hangs.
Why wont VectorMotionBlur render?
When I disable motion blur and use a separate VectorMotionBlur node, the results are MUCH better and MUCH faster when in Fusion, BUT when I goto deliver, the second it hits the Fusion comp, render fps drops to 0 and the whole program hangs.
Why wont VectorMotionBlur render?