I mean adding vector upon vector with it's direction. Like $v1+v2$ wouldn't be $[4,3,3]$ but $[4,4,4]$.
Or another example $[1,2,0]+[0,3,0]$ will be around $[3,0,0]$. I don't know math very good but in my head I can explain it as "add vector upon vector on it's rotation/direction".
The question: How to add vector upon vector on it's direction?
Here is a picture for explain my question even better: http://i.stack.imgur.com/h0xsB.png