1.

Select the Circle: Center & Radius  tool from the list in the sixth pane.

2.

Click on the point where the vector should start. A box will pop up. Enter the length of the vector.

3.

Choose the tool Point on Object . Click on the circle.

4.

Select the Vector  tool. Click on the center of the circle, and then press the point on the circle where you want the vector to go.

You have now drawn a vector of a given length.