I can't try it here but I expect I figured it out (if it's wrong, please somebody say so):
SimpleVector p1 = Interact2D.project3D2D(camera, buffer, vertex1);
SimpleVector p2 = Interact2D.project3D2D(camera, buffer, vertex2);
graphicsInstance.drawLine(p1.x, p1.y, p2.x, p2.y);