Vauxhall Vectra Headlight Malfunction

Sounds like a defective servomotor in the headlight assembly, which controls the vertical aim. Before condemning it, have you tried manually adjusting the headlight aim? Usually, there are screws on the back of the headlight housing. If manual adjustment doesn't work, or if the motor is visibly damaged, it's likely the culprit. Also, confirm light intensity from the headlight switch is reaching the headlight.

Regarding the servomotor, how can I diagnose its failure? Are there voltage or resistance measurements I can take? If I replace it, is recalibration of the auto-leveling system needed?

You could check for voltage at the servomotor connector when the system is trying to adjust, but honestly, a multimeter reading might not be conclusive. Auto-levelling uses sensors which communicate with the ECU to adjust the headlight level via the servomotor. The error memory should contain specific auto-levelling fault codes. Given the symptoms and your checks, it's pointing towards a faulty servomotor. Recalibration might be needed, but often it's plug-and-play. Best bet is a workshop with diagnostic equipment.

A technician confirmed the defective servomotor. The repair, including parts and labor, cost 1040€. Thanks for the help.