It's early and I'm sleepy so please excuse if I didn't see which Chassis this Vectra is on. If it's Chevy/Workhorse, the Chassis Power taps off the Starter Solenoid. If its Ford, Chassis Power is off an under-hood Solenoid/Relay.
A "buzzing" relay doesn't sound good to me. Either it's activation windings are shot, or the contacts aren't carrying load and go intermittent.
Don't discount internal failures in battery terminals. Those "sealed" connections can look good yet have corrosion inside.
Diagram of "Bosch" type automotive relay. 30 is Power IN. 87 is Power OUT, WHEN you have Power and Ground across 85 and 86. 87A is Power OUT when 85/86 are NOT energized. 87 is called Normally Open (NO) and 87A is called Normally Closed (NC). Not every Bosch derivative has 87A. The best brand for "Bosch" now is TYCO, but you can by knockoffs by the bunch on Amazon for $couple apiece.
Other relays may not be in the Bosch form factor, but they work the same. Some of the Bosch-style ones don't have 87A.
