It would not surprise me if there are breakdowns of drink machine branding and what they carry in various areas with info crowdfed by hobbyists. Coke vending machines do exist and some offer cashless payments via Coke on app https://c.cocacola.co.jp/app/index.html. There's an older map for the ice cold Coca Cola's slushy like drink https://icecolamap.com/#google_vignette

Some friends with kids found different varieties of not-tea not-coffee drinks on Tokyo subway platforms which we didn't see very often elsewhere, though you'll probably find more fruit-adjacent flavor more than Coke. There's also a drink (Suntory Dakara Green) I occasionally get at NewDays JR East conbiini which I rarely see at Lawson, Family Mart, or 7-11. Drugstores can be a good place to buy bottled drinks, packaged snacks, a selection of dry/retort foods, and vitamin drinks. They're often less expensive than conbini, too.