As mentioned previously, we would need a bit more context to be able to provide more guidance. I guess that what you want to do is the dot product of two vectors, say A and B, so that you can see the projection of one over another and thus you would have two vectors with the same orientation, one is not modified and the second will have a different magnitude due to the projection, if they had the same orientation the magnitude is not changed, but if they were at 90 degrees of each other, the magnitude of the projection would be zero, that is, they are orthogonal to each other. Check wikipedia for dot product:
and the dot product help for Matlab
If you still have doubts after reading these, let us know.