Vectra engine stalls at idle

Sounds like a fuel issue. Have you checked the fuel filter and fuel pressure readings? A clogged filter or low pressure could cause those symptoms.

What specific fuel pressure parameters should I be monitoring, and what's the acceptable range for a car this old?

Without the exact engine code, I can only give general advice, but it does sound like the high-pressure pump is the most probable cause. I'd advise taking it to a workshop to confirm.

A mechanic confirmed the high-pressure fuel pump was defective, causing the fuel volume regulator issue. The repair was 1510€. Thanks for pointing me in the right direction.