I did a brisket this last weekend in that kind of weather.
Left top vent open.
Had one bottom vent open, one about 3/4 open and one closed.I normally just have one open so no real difference.
Like Shane said it is the wind that effects these smokers more than the cold. Make sure you fill it to the rim with fuel. Once the temp stabilizes you can pretty much just keep an eye on it. It is amazing how consistant they burn.