Hiya,
Well i tryed few things that gave correct result. It depend of where come from your vector pass.
On what i have tryed, i use the maya shader LM2v-02 to render this pass.
I ve tryed to use it like this with the nuke motion vector but didn't find the way. So i download the Gizmo for nuke LM_2DMV to nuke on this website
here is the direct link :
http://www.fxshare.com/nuke/downloads/gizm...-Nuke-4428.html
there is a clean instruction on how to use it. It worked, even i found it wasn't a very neat blur
have a try