I overhauled the Vanos on the wife's M54 2,5i. Up to that point I had no idea it wasn't operating properly as the ECU adjusts to compensate to a certain degree.
Once put back together though, after an initial couple of minutes of rough running as the ECU tried to correct itself, it then ran much more smoothly. Over the next 150 miles or so it just got better and better.
My point being that the deterioration is subtle and you may not realise it needed doing until you've done it. Only option being to drive another with the same engine and a fresh vanos as comparison.