CAR BACKFIRED WHEN STARTED THEN WHEN RESTARTED REVED UP TO 4500RPM. IT NOW HAS NO VACUUM.

First check for codes.By no vacuum do you mean the brake pedal is hard and is the engine still racing?If so check and see if the vacuum hose to the brake booster.What you might have is carbon build up on the valves and pistons.This is a common problem with this vehicle.The problem is the motors in these cars are meant to be driven hard.So if you drive it in town and slow driving it will get this carbon build up.You need to take it out on the highway once in a while and step it down and get the cobwebs out of it.Also when is the last time it had a good tune up.Plugs,fuel filter,and air filter.G.M.makes a good top engine cleaner that they use to remove the carbon from the engine.Has to be done with the vehicle running, then the plugs have to be removed and the cleaner has to be added to the cylinders and let it sit over night.Not a thing to do in your back yard.Plus there are special directions to follow for this repair.Hope this helps.Let me know if you need more help.Good luck.