Plan B was to configure the ladder in the extension position and lean it against the vault ceiling. Upon experimentation, we decided that might be a good way to put a hole in the roof and decided not to unintentionally add a skylight. Besides, the ladder was very flexible at that length- scary to be up on.
Plan C involved a call and run over to the rental place before they closed, getting a rolling scaffolding 5x7x10 feet. After loading all the parts in Stacy's truck (and thank you for the loan), we stopped for more! paint! and headed back to the house.
Assembling the base of scaffold seemed simple, although we wished we had a 3rd set of hands. Ten we started going up, and found the rental place had given us one set at 5×7 and one at 5×5- they don't fit together.
So Plan D was to assemble a 5×7 platform 5 feet off the ground, and then put our A-frame ladder atop it.
Friends Janelle and Charlie stopped by, and they assisted us in this balancing act, and painting. Charlie's long arms came in handy up there, and I perched myself on the tippy top to get the very peak of the vault ceiling.
So the great room is now painted. I hope it doesn't need a second coat, as I don't want to do that again for 10-15 years, and by then I'll be older and frailer and hoping I can hire a team of strapping young lads to do the painting for me. In kilts, by preference.