Warranty Direct points to suspension and electrical system as causing the most trouble, and repair costs are higher than for some rivals.
Early Vectras suffered badly with wonky suspension and leaking coolant. Some rust badly, too - check under rear wheelarches. Face-lifted ones fare better all round, but knocking sounds from the engine or gearbox still signal potential expense, while squeals under the bonnet signal a dodgy water pump, but this isn't too pricey to fix.
Check the air-con units, as they're likely to fail and are expensive to fix. And, make sure the clutch is in full working order, as replacing it is a lengthy and complicated job; the parts are cheap, but the labour bill will be nasty.
The other major worry is that the cambelt will snap and wreck the engine. Vauxhall halved the recommended change interval to 40,000 miles and it's vital this is stuck to.