Not sure about the amount slope, but Leonard is absolutely correct about the function of the vent stack. The purpose of the traps in the sinks, showers, tubs,and toilets is to prevent sewer fumes from getting into the house. Any fumes should never even reach the vent system. The vent system is so liquids drain properly, and don't make all kinds of annoying gurgling sounds. Whenever they install a bathroom vent on the show, they use insulated pipes or flex tubing. I believe this is to discourage moisture from building up in the vent. They don't seem to be too concerned about sloping it. Good Luck!