0.5A draw is not likely to be the alternator. A failed diode would be pulling upwards of a few amps.
More likely to be a failed regulator or worn brushes (most likely built into the regulator), which are usually a bit easier to source than diodes.

Vectra B or C?
Reason I ask, is a Vectra C takes upto 20 minutes for all the modules to shut down, and any spike in voltage/door being opened will wake them all up again (0.5A is what I'd expect from them doing nothing, but not sleeping, as there can be upto 20 something modules in a Vectra C). To test current draw on them, you need to remove the key from the ignition, wait 20 minutes, and without disturbing/disconnecting anything, test for current draw.

Vectra B should be fully shutdown just by making sure the key is out the ignition, and all lights are off.