Much of the Roman story takes place in and around Vasio or Vaison-la-Romaine as it is now called. The green arrow on this map shows where Vaison-la-Romaine is. Livius.org has a gallery of photos of the Roman remains in Vaison-la-Romaine. This touristy site gives more background information and has many photos. The town used to be called Vaison, but was given the name Vaison-la-Romaine after many Roman remains were excavated by Joseph Sautel, the priest-cum-archaeologist Julien Barneuve meets as a young boy.
There is a useful summary of the history of the period in Chapter 10 of Bury's History of the Later Empire, particulary sections 2 (part of which covers Majorian, the emperor supported by Manlius' father -- see also DIR ) and 5.
Sidonius is mentioned in the first Roman scene of The Dream of Scipio as being under siege in Clermont (Augustonemetum). This translation of Sidonius Apollinaris' letters (scroll down if necessary) has a good introduction on his career and the life of the social milieu he and Manlius shared in Southern Gaul.