I'd try the pen tool. (open paths palette) select the pen tool click once for start point. click a second time for end point. it's one dimensional, feel fee to scale it.  click the (path selection) tool. click on your image. click the pen tool, click once for start point. click a second time for end point again on a different angle. (path selection tool) can move the two lines to line up with 2 edges. edit transform- scale... the intersection should give you a vanishing point.