Are you talking about deformation animation on the input geometry, which is scattered with a Shape operator?
If so, the only way you'll get VRay Velocity data for that, is if you convert all of your particles to meshes and render the entire flow like a giant mesh. You won't get any deformation velocity data using instances.
If you're rendering a couple of trees that shouldn't be an issue - but if you're trying to render a big forest or thousands of instances, you probably won't have any luck there (too much RAM required to render as meshes).
Alternatively you could use a tyVertexVelocity modifier on the input geometry and save the velocity data to a map channel. Then those vectors (saved in the map channel) can be rendered as instances by just applying a Vertex Color texmap to the geometry (so it won't come out in the VRay Velocity render pass, it'll just come out in a texmap override pass, or diffuse pass, or whatever you use to render the Vertex Color texmap).
If so, the only way you'll get VRay Velocity data for that, is if you convert all of your particles to meshes and render the entire flow like a giant mesh. You won't get any deformation velocity data using instances.
If you're rendering a couple of trees that shouldn't be an issue - but if you're trying to render a big forest or thousands of instances, you probably won't have any luck there (too much RAM required to render as meshes).
Alternatively you could use a tyVertexVelocity modifier on the input geometry and save the velocity data to a map channel. Then those vectors (saved in the map channel) can be rendered as instances by just applying a Vertex Color texmap to the geometry (so it won't come out in the VRay Velocity render pass, it'll just come out in a texmap override pass, or diffuse pass, or whatever you use to render the Vertex Color texmap).