Why does my Ford Focus stall after I fill it up with gas?

I bought a 2002 Ford Focus ZX3 less then two weeks ago and every time I add gas to the car it ends up sputtering and briefly stalls as I pull out of the gas station, unless I immediately press down the gas pedal and rev the engine in order to avoid it . It starts up immediately afterwards and doesn't do it under any other circumstances. Does anyone know what this could be caused by? I do not top off the gas.Why does my Ford Focus stall after I fill it up with gas?vapor lock