An apparatus for forming exterior convex vinyl siding corners for use with external walls intersecting at angles greater than 90 degrees including a vinyl siding corner member with two exterior siding members intersecting in the steady state position at 90 degrees. The exterior corner is of a flexible material such as vinyl and defines two internal slots therein within which one or more expander members is positionable to spread the exterior corner member to an angle greater than 90 degrees and approximately equal to the obtuse angle of the intersection between the first and second surfaces. The expander member may be adjustably movable to vary different angles such as would be used in the external convex corners for mounting of bay windows and other similar obtusely oriented exterior walls.