Software:
Blender Version 2.56.4 r35707

Tested on:
Desktop
Windows XP Home edition, 32 bit, SP3
Pentium4 CPU
NVidia GeForce 6200

And also on:
Laptop
Windows Vista Ultimate, 32 bit, SP2
Intel Q9300 quad core processor
2x ATI 3870 in Crossfire mode

OK, here we go...

If you have saved a render as a multilayer file, have included a vector pass, and are using this file as an image input to the compositor, the colour management option will affect the results of a vector blur operation. It appears that disabling color management causes the vector blur node to only process blur in the positive y direction when using an image file as input.

Please refer to the attached files. The .blend file is a simple scene with 4 cubes, each animated to move roughly in the positive and negative x and y direction relative to the camera. (The direction of movement is indicated in the scene.) Also included is an EXR file of the same scene, with the same frame rendered and saved in multilayer format. (The settings used to save this file have been left as is under output options.)

If the scene is loaded, and frame 2 rendered with F12, both the output window and compositor backdrop should show a successfully blurred scene. If the color management option is then un-checked, the top half of the compositor backdrop should lose the blur effect on 3 of the 4 cubes.

Other notes:
Color management does not seem to affect the EXR export, only the behaviour of the vector blur node.
Have not tested this using image formats connected into the speed node of the vector blur.



Please let me know if you need any further info.