Well, first of all - you say that "multimeter on battery doesn't match reading of VDO voltmeter" - what's the reading of both? If gauge shows some quarter volt below voltage on battery, that's normal, as there is some voltage drop between battery and gauge. The only question is - how good is your alternator working?
If you can remove exciter wire from alternator, and alternator light keeps shining, apparently exciter wire is grounded somewhere - but alternator self-excites if it's being revved behind some threshold; let's say, if engine is revved past 3k rpm - just for your info, if you continue to drive the car without this problem being solved.
As for "electronic trickery" John is speaking about - in order to rule that out, you can remove oil pressure warning relay - in VW's it's in cluster itself, but in audis of this age it's separate relay, must be above drivers feet [at least in LHD cars], with "ől druck kontrolgerāt" written on it, if you remove it, that removes oil pressure warning [both buzzer and light], but if that restores normal behavior of battery light, you'rs is faulty.
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