Vector Blur doesn't render until particle END FRAME

Hello all,

I’m rendering an animation of an object exploding (using the explode modifier and particle system).

100 particles emit on frame 108 and end on frame 113, with a lifetime of 50. I’m using the Vector blur data from a Blender Internal Scene and using that to blur a Cycles scene (Vector blur did not work directly with Cycles).

Unfortunately, the vector blur doesn’t show up in the render until frame 113 (the particle emission end frame). Is this a bug or a limitation of vector blur? Any known workarounds?

Thanks so much for any advice.

EDIT: BTW, I tried the latest build but it has the same issue (Hash 5974216).