Can we safely hang a ceiling fan from our vaulted livingroom ceiling?
That's actually what I would suggest, but be aware of the problems in opening it up. Assuming the drywall was done correctly, there's going to be corner bead. Hopefully they didn't nail it together, or you might have to replace a lot more beading that you're planning to.

You might not need more framing, though, It's probably a full box already. Just take the old box out and put a solid fan box with brace in.