Thanks Joan! And Bob is correct, I'm not sure who marked it as best answer - but given all of the information and the push-back from AirBnB super-host support, I think it may be the best answer.
The downside now is that if I forget to open a month, I wont get any bookings. Which of course is not in AirBnBs best interest.
To me, this is clearly a bug / missing feature with the new calendar, and AirBnB has said in my DM that this is just how things are now. They don't plan to add this back.
So I guess unavailable by default, and me opening up 3 months manually - while keeping my vacation days blocked - is the only solution right now.
Maybe what I can do - is make the price of those vacation days really high, that way, no one will book them anyway. (And if they do, then I guess I've made a ridiculous amount of money)