I doubt that you'd have to remove the roof to do this. Drywall will need to be removed, there is some reframing of the wall, but you should be able to do it without taking off the roof.

20x11 is a bowling alley, not a room. Bad proportions. Beyond awkward for use and for resale value. Rethink this. 20x15, leaving the other 20x25 would be much better proportions for all concerned.

"I just want to know if I need to take off the roof."

Short answer, you don't have to remove the roof.

Short pencils are better than short answers.
Leaving alone the "bowling alley" aspect.
Does the vaulted ceiling have a structural ridge?
Do you mean roof, or ceiling? Cross ties connected through the ceiling to rafters will rely on fasteners in tension aggravated by cantilever. Their type/sizing, and depth of cross tie is important and further dependent on loft loading. The wall that separates the new room from the existing, what is underneath it? IOW,how will its load be transferred?

Also think about HVAC, which can be tricky with lofts.