This should work:

#include <acknex.h>
#include <default.c>


VECTOR* Point1 = { x=5; y=5; z=5; }
VECTOR* Point2 = { x=2; y=2; z=2; }
VECTOR* vecPoint1_Point2;


VECTOR* calculateVector(VECTOR* a, VECTOR* b)
{
return(vec_sub(a,b));
}


void main()
{
screen_color.blue=150;

vecPoint1_Point2=calculateVector(Point1,Point2);
}


PANEL* Anzeige=
{
digits (10, 10, 5, *, 1,vecPoint1_Point2.x);
digits (10, 20, 5, *, 1,vecPoint1_Point2.y);
digits (10, 30, 5, *, 1,vecPoint1_Point2.z);

flags = VISIBLE;
}