The poster implied..intentionally or not..that vending machines only accepted coins.
No big deal.....
The eternal problem was the $20 bill, and how to give $1 bills in change.
You'll notice that at the self-service cashier stations at your local grocery or walmart or homedepot they have machines that give you $1s and $5s.
One huge contraption would let you feed in dozens of $1 bills, and still give you change after your purchase, but if there was a glitch in the process the system had to return your $1 bills to you.
Like I say it was huge ~ and that problem has never been addressed from the standpoint of taking those bills at that time and working with them rationally.
What has been done is the Treasury has put various security devices into the money ~ metallic threads, etc. The modern change making machines return bills because the bills can be IDd by something other than a primitive OCR.
That action actually displaced some of the marketing opportunities for the dollar coins.
In the near future that system will itself be displaced by a universal card machine (marketed by I don't know who, but it'll be there).