I seem to remember a slight involvement with that in the early '80s. Hawker Pacific replaced a lot of the structure under the floor with 4130 so a "gatling gun" could be installed by the Thais, think it was the Thai Navy. Don't know if a gatling gun is the same as a Vulcan.
Good airframe really but all that 4130 under the floor would have been the strongest part. Would have weighed a bit too.