a (gs-)vector is a array of 3 number
as example (3,5,-6)
you can access ever single number of an vector by vector.x/y/z but often you want to make the same operation with all 3 numbers, and that are the vector functions.

example for vec_set:
Code:

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var v1[3]; //this is the first vector
var v2[3]; //the second vector

v1.x = v2.x;
v1.y = v2.y;
v1.z = v2.z;
//is the same as
vec_set(v1,v2);

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example for vec_add:
Code:

---

var v1[3]; //this is the first vector
var v2[3]; //the second vector

v1.x += v2.x;
v1.y += v2.y;
v1.z += v2.z;
//is the same as
vec_add(v1,v2);

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vec_to_angle takes a normal vector and converts it into degrees

for more information RTFM!!