Were these genuine Vauxhall parts or cheapo alternatives?

Hard to say from here. Ordinarily CV joints should last the life of the car and usually only fail if the gaiter becomes split and they loose their grease or get water ingress.

The gaiter needs to be the "right stuff". The material needs to be able to withstand millions and millions of cycles of 'concertinering'. Cheap thin ones just won't do it. Buy a gaiter with a well known make name from a trusted source.

Road salt and dirt is abrasive and corrosive and can hasten failure. Giving your car a clean underneath after each winter can pay dividends.

It's also possible the gaiter has sufered mechanical damage, possibly from a flying stone or some such.

If the split has happened only recently and there's no sign of dirt and water ingress then it should be possible to refill with grease and replace the gaiter. Personally I'd remove as much of the old grease as possible and replace with fresh to ensure its clean. The joint could be washed out with brake cleaner spray.

Take it back and scream and shout at them, get the manager, shout the shop down.

Not quite get the police or feign a heart attack, but you have been ripped off, bodice, singlet and socks.

The Corsa CV gaiters are made of very thick durable material which is unfortunately quite inflexible.

Therefore they need to be fastened at both ends with a fastening which is equally strong. Vauxhall fasten them with thin lightweight one-use fastenings which are not up to the job, and just come off. After about a year.

Good strong jubilee cilps do the job but they would be too expensive, and you would never come back.