Traveling from London to Paris, then Provence, Lausanne and then back to London. Is their a Europass train ticket option or better to go 1 at a time?
Go one at a time and book as early as possible for the best cost.
London to Paris on this site select senior ticket option
https://www.eurostar.com/us-en
For travel from Paris to a city in Provence area to Lausanne and back to London use the French rail site select 60 and over and then enter your age
https://www.sncf-connect.com/en-en/
If you need to return to London from Lausanne, I would consider flying. A 1 hr 40 min flight from Geneva sure beats an 8 1/2 - 9 hr train ride. Where are you going after returning to London?