A vane arc segment includes an airfoil fairing that has first and second fairing platforms and an airfoil section therebetween. The airfoil section has a pressure side, a suction side, and an internal cavity. The first fairing platform defines suction and pressure side circumferential mate faces, forward and aft faces, a gaspath side, a non-gaspath side, and a flange that projects from the non-gaspath side. The flange extends along one of the suction or pressure side circumferential mate faces. There is a rib that has a rib section in the internal cavity that spans the pressure and suction sides and a rib extension section that extends from the rib section in the internal cavity and along the non-gaspath side of the first fairing platform. The rib extension intersects the flange to form a gusset for reinforcing the flange.