[they] briefly implemented a strategy where
one in every nine vending machine bottles
was empty ... this effectively raised the
price to 5.625 cents.
Instead, charge $0.10 but 44% of the time you get your dime back along with the bottle. Effective price is the same 5.6 cents, but people are excited to get free bottles instead of mad and disappointed.
(This might be illegal, since, it's a lot like gambling, but both versions should be equally illegal.)
They were willing to lobby for a 7.5 cent coin and update their machines to recognize that, so it sounds like they could have handled dimes. The reason they didn't want to switch to dimes is that it was too big of a price jump.
I'm curious how they eventually handled the changeover. I seem to vaguely recall Cokes when they were a dime (in the mid-sixties or thereabouts) in the machine. I also don't ever recall vending machines taking pennies or giving change in pennies--though perhaps they did. I just also have some trouble believing that Coke vending machines pretty much doubled in price over a few years.
(This might be illegal, since, it's a lot like gambling, but both versions should be equally illegal.)