A sample scene that demonstrates the correct workflow (motion vector AOV) can be found
here.
Max. Displacement
Shutter Start
Shutter End
Do Not Normalize
Camera Shutter Workflow
Transformation and/or deformation blur must be enabled.
Select the "motionvector" AOV for output (type RGB is fine).
Toggle
Instantaneous Shutter for Motion Vectors
in the
camera properties.
'Instantaneous Shutter for Motion Vectors' enabled