You really don't. Maybe those with degrees in acoustics can calculate how a vaulted ceiling with affect the sound - but I don't recall anyone being able to do it.  That's why they recommend parallel walls, because the calculations are predictable and repeatable.  You can calculate airspace in that area, and do some educated guesses on reflection direction and time delay in those reflections though.

All that said, I think vaulted ceilings are wonderful - I consider them a non issue, except in some instances where you get sort of a parabolic focusing of the sound under the peak if the angles are equal like an equilateral triangle.  

Did that answer what you were looking for?

Yeah, more airspace and non parallel walls usually means good sound. So go for it!