I made an account specifically to answer this post!
As a native french speaker, it's pretty obvious to me that the new town names are french words associated with beauty.
Lumiose: Lumière (french word for Light) & any word with the "-ose" suffix (usually associated with "large" such as in grandiose).
Santalune : Santalum (which is french for Sandalwood) and the word Lune (french for Moon, as most pointed out).
Vaniville : Either the word Vanité (Vanity) or Vanille (french for Vanilla), merge Ville (with means City or Town)
It could also be, if they follow the trend of Santalune and Vaniville, that the town names will have a plant and a french word together.