My father in law had that with his car, it is the catalytic convertor... he took it to a garage and they sorted it almost immediately. Are you finding that you are loosing power too?
the engine management light does not come on for any reason, it comes on as there has been a fault code logged in the ecu indicating a problem, most of the time it relates to a fault which results in irregular emissions. it does not reset itself it needs to be scanned and cleared by diagnostic equipment. if it is the latest shape vectra then only vauxhall has the diagnostic gear to scan it as its single wire.