The limit is 30 armor against weapons with penetration of 1 (if I am not mistaken above 3%).
The chances are so abysmally low with armour above ca. 15, that it doesn't really happen. Combine that with the relatively high health of vehicles compared to infantry damage, and you see why it quickly becomes entirely irrelevant, unless infantry is capable of penetrating every single or every second shot. Missing is also almost entirely negated, due to the combination target sizes and infantry accuracy.
So the Puma basically doesn't have to worry about this, and (as written above) the 251/17 and the M15A1 fulfil different roles anyway. Not that the 251/17 couldn't use some sort of adjustment, but this shouldn't come in the form of armour buffs.
The 222 on the other hand would actually work better in its intended role if it had higher armour (and less health to even it out against other vehicles). It could then more reliably fight infantry, while staying effective at finishing off light vehicles. It would of course lose the ability to outright play "hunter/killer" if used in pairs, but that strategy was stupid from a balance perspective in the first place.