What kind of vent pipe is it? If it's the plastic, corregated stuff, throw it away and replace with solid pipe, either aluminum or galvanized. How long is the run and how many elbows? What's likely happening is the air is full of moisture, it is slowing down and lingering in the vent long enough so that some of it is condensing, running down the vent onto the floor. If there are any obstructions (clogged line, too many elbows, rough from the cheap plastic, or lots of lint in the pipe), it slows donw and isn't staying a gas.