Select all vertices on the portion or side of the object that will be the center of symmetry. Next, press the CTRL. Key and Mouse Click to move the Drawing Plane to the selected vertices. Deselect and then re-select all vertices and polygons and lastly, apply the Duplicate with Symmetry command to create a mirror object on the opposite side of the Drawing Plane.
How do you do a Boolean Subtraction in the Vertex Modeler? In the vertex modeler under the PolyMesh menu, there are 3 Boolean operations. When you select two parts of the vertex model which overlap, you can see the first two highlighted. However, the last one (Boolean Subtraction) remains disabled. How can you get this to be selectable, since you want to cut object #2 out of object #1 like in a boolean subtraction? There is indeed a Boolean subtraction in the vertex modeler. But before you can select this mode, and after you've selected both parts, you also need to indicate which of the two parts is the one from which the other needs to be subtracted from.
You do this by clicking a vertex of that "from" object in a special way: You need to select a point of the mesh you want to substract from using Ctrl+Alt+Shift and click the vertex (the point will turn yellow). Thereafter, the PolyMesh menu will show Boolean Subtraction enabled and available for selection.