1991 Pontiac Grand Am Rough idle

I took my car to WalMart to get an oil change, when I got it back the staff member said they couldn't find the oil filter but had already dumped the oil so she didn't charge me for the change but said I should bring it back or take it somewhere else to do an oil change/filter change. When I went to start the car it idled horrible, terrible rattle and the car stalled but I wasn't suprised. It is a special car sometimes. It's still doing it and now the check engine light comes on about 30 seconds after the car starts with an error code 33 "MAP sensor signal voltage was too high (low vacuum) for at least 5 seconds when throttle opening was under 2%". It never did this before the oil change and I get oil changes every 3,000 miles. Someone suggested the person could have disconnected a vacuum hose and forgot to reconnect it while in search of the secretive and illusive oil filter but I want to be relativly sure before I take it back to them and say what's going on.