I would start with ( sorry no pun intended ) an RPM starter check & current draw test & battery load test. Its possible if the car has suffered with starting issue's in the past the starter is worn and drawing too much current, this will effect the instruments with voltage fluctuations and turn over too slowly causing failure to start.

Usually with vectra's its leak off pipes, injectors or filter housing causing air leak issues so this must be checked to make sure its perfect, just because they are replaced doesn't mean its fixed. there are some modified seals as well for the injectors.