Why does my car die out when not using the gas?

When I start the car I most of the time need to press the gas to make it start. Most of the time when i'm driving and not on the gas it will putter and die out unless I give it gas. Samething when at a stop or parked. I replaced the fuel pump, gave it a tune up, changed the coils...but still nothing and I was finally told it was a vacuum hose...ok...which one and how can I change it.