Help with the roll angle

I have an animated model of a warrior and a static model of a helmet, and now I'm working on a function to recalculate the position and angles of the helmet every frame. I found only this code:

function addons(ENTITY* entity1, var v1, var v2, var v3)
{
VECTOR hand_pos;
VECTOR hand_pos2;
VECTOR hand_pos3;
VECTOR hand_pos4;
VECTOR vec1;
VECTOR vec2;
VECTOR temp;

vec_for_vertex(entity1.x, Player1,v1); //vertex to place origin at
vec_for_vertex(hand_pos, Player1, v2); //vert to orient pan and tilt
vec_for_vertex(hand_pos2, Player1, v1);
vec_for_vertex(hand_pos3, Player1, v3); //vert to orient roll
vec_for_vertex(hand_pos4, Player1, v2);

vec_sub(hand_pos, entity1.x); //sets our pan and tilt
vec_to_angle(entity1.pan, hand_pos);
entity1.roll = 0;

//all this does the roll, courtesy of Gnometech
vec_sub(hand_pos3, hand_pos4);
vec_set(vec1, nullvector);
vec1.y = 1;
vec_rotate(vec1, entity1.pan);
vec_set(vec2, nullvector);
vec2.z = 1;
vec_rotate(vec2, entity1.pan);
temp.x = vec_dot(vec1, hand_pos4);
temp.y = vec_dot(vec2, hand_pos3);
temp.z = 0;
vec_to_angle(entity1.roll, temp);
}

Angles pan and tilt are working fine so far, but roll isn't right calculated. This problem is getting me mad and I can't find another solution. Can somebody help me??

Thanks!

There's an attach code in wiki and in AUM resource.
You could always search there...

It should work. Do you give to the function 3 vertexes? They should positioned like a "L"

Yeah, the three vertices you pick on your parent model are important. They should form an "L" shape as Widi mentioned. If they are in a straight line (or close to it), then the roll part goes all crazy.