Quote:

...for instance you are using local variables and then return them, but they cease to exist outside the function and thus all you return is garbage.



Are you sure about that? Where do the local variables leave the function?
If I exclude the line " vec_sub(Copy_b,a); ", then vector Copy_c is returned properly. I ask again what has this line to do with vector Copy_c ?

Perhaps I really misunderstand something very badly, so please be patient - If I had found a tutorial about vectors I would have worked this through...but I was so enthusiastic because when I used usual variables instead of vectors everything worked perfectly.