Your function code should be using posidon[0], posidon[1], and posidon[2] instead of posidon.x, .y, and .z
Unless you replace its global definition as
VECTOR* posidon = { x=0, y=0; z=0; }
instead of
var posidon = vector(0,0,0);
Using the var version may cause odd problems later if you use the vector(x,y,z) function
too often.

Using the VECTOR* posidon = { x=0, y=0; z=0; } and leave the rest of your code as is would
be my preferred option.