This answer to What is the difference between the Vulcan 562 configuration and the Vulcan 5H2 ("Heavy") configuration? explains that the 2nd stage nozzles were modified by extending them, which appears to allow for more expansion (they almost touch in the drawing) and links to https://www.ulalaunch.com/docs/default-source/rockets/vulcancentaur.pdf?sfvrsn=10d7f58f_2
I don't see any additional distance in the interstage area, they could have been this long from the beginning.
That makes me wonder how improving the expansion for a vacuum engine could have been an afterthought in this case. Why not make them longer from the beginning?