yes you are grin

In the case your cubes are perfect, there is a simplified way to compute vertex positions.

Code:
90 degrees rotation
x' = -y;
y' = x;


so you should draw your square as follows

Code:
draw_line3d(vector(vMax.x, vMax.y, vMin.z),NULL,100);  
draw_line3d(vector(-vMax.y, vMax.x, vMin.z),drawcolor_vec,100);
draw_line3d(vector(-vMax.x, -vMax.y, vMin.z),drawcolor_vec,100); 
draw_line3d(vector(vMax.y, -vMax.x, vMin.z),drawcolor_vec,100); 
draw_line3d(vector(vMax.x, vMax.y, vMin.z),drawcolor_vec,100);


Salud!