In croatia they have tiny little 5'x5' walk up convenient stores everywhere that might as well be vending machines, but I'm sure the drunks would get in there in no time if they weren't manned.

Railway stations in the UK used to have cigarette vending machines, back before smoking ban.

Not sure about the UK but in the US when there used to be cig vending machines it would be under the supervision of staff, so like in a bar.

In Japan they are out in the open in non descript corners and alleyways, although there is usually a streetlight.

They do require a "tobacco passport" to operate, which aims to make it harder for kids to buy them: https://en.wikipedia.org/wiki/Taspo

Then again, this is the same country where it's sufficient to self-certify by tapping a button that you're over 18 when buying hard liquor at the 7-11.

Is underage drinking a problem in Japan? Genuinely curious, I know salarymen feel obligated to drink with their bosses but have never heard about unruly drunk Japanese teenagers

Drinking is allowed over 20 years old but some university students tend to drink on eighteen on party. It's illegal but not thought as serious problem.

For high school student or below, smoking is problem (So vending machine must have authenticate feature like card(Taspo) or face age recognition) but drinking is looks like not popular but still considered as problem.