FFX riddles

[Shiny]

Perhaps, the only true purity
Is that in which used to obtain serenity --> i'm gonna guess this is the "no encounters" ability
For sooth, it happens to be salinity --> purifying salt i'm guessing.
That forsakes all in this vicinity

unless it's the fallen monks in zanarkand, then maybe it's a particular piece of equipment with no encounters on it. sure wish i knew all the names :S

You are correct with your reasoning, but there is a specific name for the answer to the riddle. Clues are in the last words of each line.

although it does say "in this vicinity" which indicates that it's a place...

That's speaking generally. It's not a specific place.