There is an ancient part of code floating around (at least 4 years old) where I answered a similar question. Yes, you need a third vertex for the roll and do some basic vector arithmetics.

Here comes the code, try to put it to good use. :-)

Code:

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function addons(entity1,v1,v2,v3);		//function prototype

entity* tempentity
var hand_pos[3];
var hand_pos2[3]; 
var hand_pos3[3];
var hand_pos4[3];
var vec1[3];
var vec2[3];

function addons(entity1,v1,v2,v3)
{
//	you = me;
	tempentity = ptr_for_handle(entity1);		//entity to attach to me
	
	vec_for_vertex(tempentity.x,you,v1); 		//vertex to place origin at
	
	vec_for_vertex(hand_pos,you,v2); 			//vert to orient pan and tilt
	vec_for_vertex(hand_pos2,you,v1);			
	vec_for_vertex(hand_pos3,you,v3);			//vert to orient roll
	vec_for_vertex(hand_pos4,you,v2);			

	vec_sub(hand_pos,tempentity.x); 			//sets our pan and tilt
	vec_to_angle(tempentity.pan,hand_pos);
	tempentity.roll = null;
	
	//all this does the roll, courtesy of Gnometech
	vec_sub(hand_pos3,hand_pos4);
	vec_set (vec1, nullvector);
	vec1.y = 1;
	vec_rotate (vec1, tempentity.pan);
	vec_set (vec2, nullvector);
	vec2.z = 1;
	vec_rotate (vec2, tempentity.pan); // those vectors span the plane
	temp.x = vec_dot (vec1, hand_pos3);
	temp.y = vec_dot (vec2, hand_pos3);
	temp.z = 0; // if target is not in the span of the front vector, temp is not equal nullvector 
	vec_to_angle (vec1.pan, temp); // we re-use v1, as it is no longer needed
	tempentity.roll = vec1.pan; // As temp.z is 0, only the pan of v1 will be set and that is our desired angle 
} 

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Regards,
Gnometech