Hi, i'm checking this issue and it seems we have to do several changes in the V-Ray shader, to support motion blur and velocity channel on animated objects.
Apologies by that. We'll prioritize this task, so it's available for the next Forest update.
Until then i'm afraid there is not any workaround, except convert Forest to instances using Forest Tools
Since Forest Tools cannot translate the random animation offset, i suggest to create several custom objects with different offsets, and randomize them in Forest. In this way, the result will be identical once converted to stand-alone instances.
I hope that helps.