hallo i have a problem white this code

it trace but i dont get the vertex
fisrt time i try this function because i am going to use thise white vec_to_mesh so i realy need that vertex_search
i tried to set vec_set and more but getvertex stays -1 watever i tryed
sombody a idee thx

var getvertex;

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