Hi, unfortunately there is not a simple solution for this problem.
As you pointed out, V-Ray requires that motion vectors are defined explicitely. But in Forest motion depends of the position of each item at different time intervals.
That means we should compute the full distribution at least two times: one at the current frame, and other at the next frame, to get the difference between them.
That effectively doubles the processing time, besides adding an extra level of complexity to the Forest workflow (which is very complex already).