bl_info = {
    "name": "Resymmetrize Vertices",
    "author": "Markus Ewald",
    "version": (1, 0),
    "blender": (2, 7, 7),
    "location": "Object -> Tools -> Misc",
    "description": "Saves and restores vertex selections",
    #"warning": "20 Nov 2016",
    "wiki_url": "",
    "tracker_url": "",
    "category": "Mesh"
}

import bpy
import bmesh

def sgn(x):
    return (x > 0) - (x < 0)

class Resymmetrize(bpy.types.Operator):
    """Resymmetrize selected vertices"""
    bl_idname = "mesh.resymmetrize_selected_vertices"
    bl_label = "Resymmetrize Vertices"
    #bl_options = {'REGISTER', 'UNDO'}  # enable undo for the operator.

    def execute(self, context):        # execute() is called by blender when running the operator.
        bm = bmesh.from_edit_mesh(bpy.context.active_object.data)
        selected_verts = [v for v in bm.verts if v.select]

        # ?! str(selected_vert_count) returns string, report() wants something else, it seems
        selected_vert_count = len(selected_verts)

        if len(selected_verts) != 2:
            self.report({'INFO'}, 'Select exactly 2 vertices before using this operator')
            return {'FINISHED'}

        average_x = (abs(selected_verts[0].co.x) + abs(selected_verts[1].co.x)) / 2
        selected_verts[0].co.x = average_x * sgn(selected_verts[0].co.x)
        selected_verts[1].co.x = average_x * sgn(selected_verts[1].co.x)

        average_y = (selected_verts[0].co.y + selected_verts[1].co.y) / 2
        selected_verts[0].co.y = average_y
        selected_verts[1].co.y = average_y

        average_z = (selected_verts[0].co.z + selected_verts[1].co.z) / 2
        selected_verts[0].co.z = average_z
        selected_verts[1].co.z = average_z

        bmesh.update_edit_mesh(bpy.context.active_object.data)

        return {'FINISHED'}            # this lets blender know the operator finished successfully.

def register():
    bpy.utils.register_class(Resymmetrize)

def unregister():
    bpy.utils.unregister_class(Resymmetrize)

# This allows you to run the script directly from blenders text editor
# to test the addon without having to install it.
if __name__ == "__main__":
    register()