You can use 3D trees with Forest Lite, it has the same rendering capabilities than Forest Pro. We have included a sample billboards library, but you can assign your own meshes or proxies assigning them as Custom Objects.
Please, see Urban Park tutorial. The scene includes some 3D tree samples and can be completed with Forest Lite:http://www.itoosoft.com/english/menu.php?id=forest_tutorial_urbanpark
Forest works as an advanced scattering tool, but it has not effect in render times. We have rendered scenes with millions of trees (each one with 1 million polys), getting low rendering times, so 100 trees must not be a problem for VRay... how much time take your scene to render ?
BTW, Forest Pro library doesn't include 3D objects either, only billboards. You must use your own models.