I have a stop-motion video I am working on, and I want to add motion blur to the character of the video. The character is the only thing that moves, yet if I use the Vector Blur node in the compositor, it applies a blur to every object. If I set min-blur up too high, it doesn't allow the character to have motion-blur, but it I have it too low, it makes everything blurry. Is there a way to fix this or do I need to use another node to add motion blur? I am trying to simulate what After Effects does when you add the Motion Blur effect to footage. Here's an image of my trouble. I have used a split-viewer so you can see the difference between original and current motion blur settings. (You'll have to view the image larger, you can barely see it in the small preview)
I am afraid that there is no optical flow available in Blender at this time. Optical flow technology uses motion vectors to estimate motion vectors across an image.
To simulate something similar try animating a pseudo mask object that follows the moving 2D image. You can animate it manually or you could use the tracker to follow elements of the animation then hook an object to it in 3D view. Take these actual object movements to use as vectors for the compositor.
For a better result, you can separate the character from the background if you have a clean plate of the background using a Difference Key node.