I would recommend the less reduction possible. What XFrog will do is reduce the number of vertices in each spline (XFrog is spline based). If your trunk has 9 vertices, the resulting mesh will be alright, well rounded, but if it is reduced to 5, then the trunk will look like a pentagon, not good for a trunk, but acceptable for a small branch. Since you won't decide where to lighten up the model with this slider, I recommend very low changes. But if you know your way around XFrog, check yourself how many points a component has, and reduce it when possible.
But  I'm not sure this is even necessary. Once you did your material tweaks in Vue, save the model as a vob, then you should have no problem with it.
The poly count XFrog gives you is the poly count you'll get in Vue, since Xfrog uses triangles instead of quads,  just like Vue. Some HD Vue trees are in the 400k polygons already, so you should be alright with the Xfrog models, once saved as vob.