Directional Vector:

\[
   \underline{v}
   =
   \overrightarrow{AB}
   =
   \overrightarrow{OB}-\overrightarrow{OA}
   =
   \begin{pmatrix}
       x_b-x_a
       \\
       y_b-y_a
   \end{pmatrix}
   =
   \begin{pmatrix}
       v_x
       \\
       v_y
   \end{pmatrix}
\]