Making Physijs take vertex shader displacements into account

Question

I'm not sure if the title is the best, but I'll try to explain what my problem is. I'm playing aroung with Three.js and Physijs to make a simple game where the player can walk around on a field. I'm using a plane geometry as the ground and wanted to use a displacement map to change its shape. I previously manipulated the vertices directly (using just some mathematical formulas to displace the vertices) and used a HeightfieldMap for collision detection and this worked great, but now that I implemented displacement mapping through a vertex shader, Physijs treats the ground as a flat object. I'm guessing the manipulation of the vertices in the shader happens after the Physijs simulation, but is there any way to get Physijs to take the vertex displacement from the shader into account when doing collision detection? Alternatively, is there a good way to do displacement mapping without shaders?