Try using Ignore_me in the trace, it is possible depending on the models center that you are tracing the wrong model. Also you are using use_box...from the manual:

GET_HITVERTEX When a model is hit, the number of the hit vertex will be returned in the hitvertex variable. Mutually exclusive with USE_AABB.

var getvertex;

while(1)
{
c_trace(my.x,vector(my.x,my.y,my.z-100), ignore_me|use_aabb|get_hitvertex );
getvertex = hitvertex;
wait(1);
}