There was nothing particularly wrong with the Signum (we had one as a hire car once and it had a lot of space and all the strengths of the Vectra) but the idea was deeply flawed.

They managed to turn their backs on the main customer base of the Omega (the police) and also tried to compete in the executive market with a car whose Vectra origins were obvious. It had the double whammy of being a hatchback and a Vauxhall which meant it struggled to sell to image concious buyers. It might have done well if they priced it closer to the Vectra but they charged a large premium.