Use the second measure bar to define the depth dimension on one side to mp1, the measure point on the auxiliary horizon line parallel to the measure bar used.

When one end of the measure bar is aligned with the anchor point, the back corner of the cube is located where the line from the other end of the measure bar to mp1 crosses the bottom left edge of the figure. Mark this point.

Again aligning your straight edge with vp3, draw a line from this point to the top left edge line :

This is the back vertical of the cube. Connect the ends of this vertical along vanishing lines to vp2. These lines define the back left upper and lower edges of the figure.

Finished drawing