1998 grand prix

VACUUM PROBLEMS USUALLY CAUSE IDLE PROBLEMS NOT A COMPLETE SHUTDOWN. SOUNDS MORE LIKE A FUEL PUMP RELAY.

if the check engine light is not on as it idles more than likely the cause is one of 3 things ,a vacuum leak this will cause higher idle speeds and loss of idle, the Idle air control sensor or switch stuck or not functioning this will cause no idle but if you depress the gas pedal slightly the car will run and idle as long as your foot is on the gas, last possibility is air inlet leaks these cars mostly came with mass airflow sensors if the tube that connects this sensor to the throttle body is leaking or broken it will cause the problem your explaining however this is the rarest of the 3 causes

check your codes and vacum hoses and see if it needs a tune up

Try a new gas filter and have the fuel pressure checked. Scan the engine computer for trouble codes. Check your air filter and replace if dirty or old. How old are your spark plugs, need new ones?
Maybe it just needs a tune up.

TPS - throttle position sensor .

chunk of aliminum in cylinder. checdk plugs for dammage

Check the mass air flow sensor on the throttle body. It may need cleaning or replaced if the filament is broken.

provided you don't have a vacuum leak, (check all hoses for tightness and cracks with engine running) it's probably your idle air control valve or sensor.

engine mounts?

check all fuses for turn signal and brake lights.change fuel filter . first. do a little tune up change spark plugs and wires . if changing fuel filter dont help. have fuel pressure check first before buying a new fuel pump .they run after taxes 400.00 dollars. your fuel pressure should be 41 to 47 psi. replace fuel pump relay also.