On a vaulted ceiling with the planks running long, you would nail directly to the rafters; no need for furring strips.

You don't need furring strips if you run the planks perpendicular to the ceiling joists or rafters in the case of a vault. You do need them if you install the planks parallel to the joists/rafters.

Ok great. So if I run perpendicular to rafters, is it ok if I just use adhesive on tongue and groove boards and nail them with 2 inch Brad nails onto the tongues to conceal the nails?

You need drywall as your first layer.

Yes, you need to have a layer of drywall under the planks for fire protection. No need for adhesive. I would snap a chalk line over where the rafters are so the plank nails hit solid wood.

Great! Thanks all. I’ll skip the adhesive