Don't know the year of your Vectra, but we had a 97 Adventurer and the toilet had a fingertip size vent hole in the back of the bowl just below the rim. It was a direct vent down to the tank. Contacted the toilet manufacturer and they sent me a plug for it which solved the same "stink" problem you had. Probably not too many of these toilets around anymore.
Barry & Sue Miller("extended" RVer's, 6 mos + per/year) combined trips.
04 Journey 39K, C-7, 330 hp, towing: 13 Ford Edge AWD, or 19 GMC Canyon 4WD, ReadyBrake.