Top vent: closed. If you use foil on the top completely effectively the vent is blocked off anyway. If you have a steel plate on top covering most of it, the kettle lid's vent is not effective. I am toying with the idea of using flashing to seal of the gap between top disc and the kettle lid to see what difference that makes. I could use foil, but the point of getting the disc was so i don't have to bother with foil. Plus you can cook on the SS disc, you can do teppanyaki or any other flat surface cooking (on other occasions of course! not advocating that you do both at the same time, the kettle pizza demands that you manage your fire!)
Bottom Vent: Open, you need air to come in so the coals will reach flaming heat. If you close it, your main source of air is the mouth of the insert. In draughty days, you get a natural bellows effect on a fire from the up draught, you 'fan the flames' as it were.
There is another kettlepizza thread on this forum that Elijah started and I have a pic of a 90 second bake there. I recall it was a windy day.
edit: here it is http://www.pizzamaking.com/forum/index.php/topic,19017.20.html