I think the question is really why? Are you trying to make a small house feel like a large house? Instead of dealing with multiple vaults and other issues (e.g. can't do crown, have to add beams, need special eyeball spots instead of recessed lights, need flat spots for ceiling fangs, maybe adding windows at the peaks) why don't you just do 9' in the entire house? It will give you that spaciousness feeling you're seeking without being out of scale for that size.

It's an open concept plan?

In my area there are many homes open concepts that vault the entire living area. It's usually fine in the family and dining room but the kitchen is usually a disaster.

Go with 9' ceilings. We did and love them. But if you do vault avoid the following pitfalls. These are 3 different homes in my area.

Go with nine foot high ceilings. Tape joints other than 90 degrees and 180 degrees and 270 degrees are a train wreck waiting to happen.

one thing I would personally consider if doing vaulted ceilings, is the roof insulation value going to be the same as flat ceiling?

blown cellulose is a lot cheaper than foam insulation boards at the roof.

you also have to consider that vaulted ceilings mean a sealed attic. It could be a big consideration depending on what climate zone you live in I think

Lower ceilings are more human scale and more comfortable. Friends that live in houses with very high ceilings find that they often migrate to the smaller rooms with lower ceilings. FWIW, we're doing 8'5" in our 7,000 sq ft house. The only 'vaulted' ceilings in our house are the loft/attic but these are about 8' at the peak and slope down from there.

It depends on the room. Frankly, I'd have just left all the ceilings at the maximum height the framing would support but my wife and the architect had other ideas. The ceilings are constantly varying heights, lower over the dining table, over the sofa, higher at other places (like over the master bed). In the exercise room we went as high as possible (to the point of leaving the central beam exposed). It's helpful to walk it at the last minute. While the lowered ceiling in my bar/TV room looked good on paper, when I saw the framing it looked too cave like. I had them raise that back up and I'm happy I did.

Would you be achieving vaulted ceilings through rafters or vaulted trusses? If via trusses, your achieved vault on the inside would be only 3:12. Assuming a standard bedroom is 12' deep, you would then have 8' tall ceilings at the exterior and 11' at the interior, with an average height of 9'-6". So you probably would achieve a similar, if not a more successful effect using 9' walls and by making each of the windows 6"-12" taller to let in more light and open the house further to the outdoors.

We did live in one similar sized house like this where every room was vaulted including the bathrooms. I think it did make the public rooms feel more airy, but the bedrooms were small enough that the vault didn't have a huge effect. Because you entered each room on the tall side, your eye mostly read the wall and window height at the exterior wall. But if the ceiling could be emphasized with beams or glass high up in the gable end walls, I think it could be made more effective. In short it's really all in the details and how the design is handled to make the vaulting worthwhile.