Good question...and one I agonized over for quite a while before I settled on a pretty easy solution. Basically, I build my vessels in two (or more) parts. I essentially build each half as a separate vessel, and turn both the inside and outside. Then, I double stick tape the two halves together (or press fit them together with my tailstock) and refine the outside shape so it is seamless and a smooth curve. I put several pencil marks on the outside, so I can reassemble the halves in the right orientation. Then I take the two halves apart, and finish turning the inside of each half. That is where it gets tricky, because the wall thickness needs to be very close, so any inside seam is not noticeable. Although for narrow necks, where the inside seam is inaccessible, it's not an issue.
Then, when wall thickness is what I want, I glue the two halves together, and finish the outside...and inside, if it is accessible. I never use a mortise joint. After all, if I had turned it as a single vessel, there would be no mortise anyway.
