I reported this yesterday but it was incorrectly closed as a duplicate of a different issue. I cannot add notes to the now closed issue so I have clarified the difference in the bugs and re-reported here.
This issue is NOT a duplicate of 0015059. I'm aware of that issue and have experienced it myself. The 0015059 bug is that you cannot ENTER optics mode. The bug in this instance is that you can ENTER the optics mode fine but you cannot change the variable zoom level. This is a different bug.
The steps to reproduce below have been updated to clarify this. This has been confirmed for the Slammer and the Marshall at least, so I suspect it affects all 3-crew armoured vehicles. Affects both singleplayer and multiplayer.