Just while you're waiting for the others who may have a better solution, here are a few suggestions from me to get the ball rolling as a former Vectra B owner myself, and boy did I spend a lot of my time under that car, the neighbours thought I lived under it...

Cheapest perhaps - 99p Store (not Poundland) they sell 6 or 9 pack of waterproof resin, each about the size of a 5p coin on a blister pack, assuming using it doesn't partially block the pipe then it might be a safe solution but the pressure may force it out if used alone. There may be a risk of damage to meters/sensors in the engine if the stuff gets into it I guess.

You might like to use the above in combo with a cut piece of flexi hose pulled over the crack or other rubber product such as a rubber glove finger or more rigid/thick bit of plastic or rubber. Or you might be best off just using this and avoid the risk of any resin damaging a sensor.

There's probably a more expensive epoxy resin product out there especially for high pressure sealing, but I can't think of a brand. A motor factors place may have a suggestion - where I sometimes go he's like a little boffin that has picked up tips from scores of car mechanics.