SIMON is a 3-D vehicle dynamics simulation program with an option to use the  DyMESH 3-D collision model.  You have set up a SIMON event, which will simulate the trajectories of the vehicles based upon  the conditions, but in order to have a vehicle-to-vehicle or vehicle-to-environment collision, you need to turn on DyMESH.  On the menu, select Options, DyMESH and then check the box for Use DyMESH.  If you need to have the vehicle body interact with the terrain model, also check the box for  Include Environment.  When you run the event now, DyMESH will continually look for  interaction between the mesh (geometry file) of one vehicle versus another vehicle, or against the mesh of the terrain model, and will properly calculate the collision forces and moments and pass them to SIMON.