confused

94 4.3 270K miles tbi G20 van. Slightly off idle stumble/hesitation under very slight load,like backing in to a parking space. when in park very light throttle pressure will cause a rough almost missing out type of vibration. Enough that I can see the steam in the exaust puff. Normal idle speed and performance. If it is a vacume leak I think it would idle at a higher RPM. If an ignition problem I would think there should be a miss. HELP!