What I tried yesterday.

I tried some struggle for understanding.

1. Just try, select four top vertices and make 'vertex group' on a cube.
2. Deselect all vertices.
3. Open the 'report' window.
4. In a 'Vertex Group' tab, push 'Select' button, of course the four vertices of the group was selected. See the report window, I could see 'bpy.ops.object.vertex_group_select()'.
5. Copy it and paste to the script window.
6. Deselect all vertices again for test.
7. Push 'Run Script' button.
8. On the screen, four vertices were selected!

It's a very little one step for me.  :-)