To place a vector on the screen, click the left-side mouse button anywhere in the viewing grid.  Clicking a second time places the second vector within the grid.  

Once both vectors have been drawn, their orientation and length can be changed by placing the cursor over the head of the vector, pressing down on the left-side mouse button, and dragging with the mouse.

The vectors can be added by selecting the A+B button and subtracted by selecting either the A-B or B-A buttons.