If it’s the 6 speed, then you’ve got the F40 gearbox (nothing to do with the Ferrari with the same name!).
They’re not the greatest gearboxes in the world – they’re fitted to Alfa 159s and SAABs too, and they’re not amazingly slick changes. The good news is that lots of the SAAB lot will be running 250bhp+ and 370NM+ through them and you don’t hear of them failing – the clutch will go first.
The first port of call is to change the gearbox oil. GM issued a service bulletin to reduce the amount of oil in the gearbox and put fresh in, ending up with 3.something litres of oil in the gearbox. How to do the gearbox oil change is documented on pretty much every Vauxhall and SAAB forum there is. The SAAB forum types all like the Redline MT90 oil from Opie Oils.
The other port of call is that your linkage / turret needs adjusting. On the SAABs the selector turret a known wear point with specific repair kits. There may be a similar setup on the Vectra.
The final thing is that it’s an F40 gearbox and you’re unlikely to get a much better one. Learn to double clutch (ie, clutch down, go into neutral, clutch back up and into the next gear) and you’ll at least avoid crunching it 🙂