It's any form of intentionally attempting to break into armoury which should be KOS. That is why entering Tardis is KOS because once you've entered, you have access to armoury bit hitting one button. In the same way, you have access to armoury by hitting the vent. In the same way you have access to armoury by door blocking it.
Furthermore, I don't believe that by changing the rule to make hitting the vent on top of armoury KOS, the game will be any less fun for reds. It will not stop some of them from even breaking it. If anything it will make it even more fun as rebellers now have to come up with ingenious and sneaky ways to get ammo. Breaking the vent on top of armoury is easy and the round just gets boring and repetitive once every red has gone in to get ammo. I'm saying this from experience as in my old community, hitting the vent was KOS and reds started to find sneaky ways to break the vent or find other ways to get ammo. Which just made the game a little more fun IMO.