First, there is nothing unique about a vector being normal to another vector unless you are in two-dimensional space. Is this in 2D?
Second, I think you can do much better at stating the question in the second sentence. What "lines" are you talking about and what do you mean by "delimiting" those lines? Please try to explain this in much greater detail. You are much more likely to receive a satisfactory answer if you do. Perhaps you can give a concrete example. Otherwise the best you can hope for are wild guesses.
