I was at Au Lac last month and I heard from a friend that there is in fact only a limited selection of real vegan choices and others only vegetarian. He wasn't sure about fish sauce in the vegetarian stuff either. But still, there is definitely a vegan selection and one can do nothing wrong by ordering the choices displayed in pictures behind the counter, they are all definitely vegan.
I had Pad Thai (not thaaaaat cheap with 16 zl but definitely filling or even for two people) and some spring rolls, both well made and tasty.
As for the rest, small place, but a good choice if you are in Chmielna and looking for some asian food without speculating too much if it's vegan (like in Du-za-Mi-ha) - at least as long as you stick to the special items. If you don't care about asian though, I'd suggest walking around the corner to Tygrys for some burgers and hotdogs.