Vectra c's are supposed to be self adjusting
Vectra b have an adjuster wheel inside the drum so you do it there and under the handbrake lever inside the car
This is from the vec c. possibly the bits arrowed would be for adjusting the tension on the handbrake.
It could be faulty if not self adjusting.
i have the vec b so its a little different. Which has a wheel inside the drum
Other than that i don't know what advice to give.
I'm sure vec c has discs and not drum so i don't know how it works