A vane arc segment includes a continuous airfoil piece that defines first and second platforms and an airfoil section that extends between the first and second platforms. The airfoil section has a pressure side and a suction side. The first platform defines axially-sloped suction and pressure side circumferential mate faces, first and second axial sides, a gaspath side, a non-gaspath side, and a flange that projects from the non-gaspath side. The flange extends along at least a portion the axially-sloped suction side circumferential mate face.