A vane arc segment includes an airfoil fairing that has a platform and an airfoil section that extends therefrom. The platform defines periphery edge faces, a gaspath face, and a non-gaspath face. The platform has a flange that projects radially from the non-gaspath face. The flange defines a forward flange face, an aft flange face, and a radial flange face. The flange has an end portion adjacent one of the periphery edges. The end portion includes a step that defines a radial step face that is radially intermediate the radial flange face and the non-gaspath face.