If I install the latest vassal will I be able to use files for Pbem games that are not the latest. I have 3.2.17 right now.
In the vast majority of cases, yes. The rare exceptions will be if you are using any of the small handful of modules known to use techniques that are no longer supported in VASSAL 3.4.x.
It’s quite easy to keep VASSAL 3.2.17 installed side-by-side with the current version if you find you need to do that.