//from inside OnVertexMoved
            IFeature source_feature = ArcMap.Editor.EditSelection.Next();
            IEditSketch e_sketch = ArcMap.Editor as IEditSketch;
            IGeometryCollection gc = source_feature.Shape as IGeometryCollection;
            IGeometry geo_part = gc.Geometry[e_sketch.Part];
            IPoint orig_point = (geo_part as IPointCollection).Point[e_sketch.Vertex];