Hair + vector blur ???

Hi !

I am currently working on a project, a short animation made with blender.

But I have a real problem: I made a character who has hair (made with the particle system), and a composite node “vector blur”. When I render the scene, the hair is not affected by the vector blurring. Why ?!?!

I have spent a lot of time to find a solution on the web before creating a new thread here!:o
Thank you in advance for your help !!