Sum and difference of two vectors - The parallelogram method

You can view the animation that shows the sum and difference of the vectors displayed using the parallelogram method, simply clicking the related buttons. To show just the vectors, click the Restart button. At the end of the animation, move the green points that define the vectors positions to explore how their sum and difference modify accordingly. Which reciprocal position do they need to take in order to make the parallelogram method not applicable? In this particular case, how can you do to calculate the sum and difference of the two vectors?