Paraguay wasn't a buffer to anything. They survived prior and after the war simply because it wasn't strategic territory, basically residing in the middle of no where. Uruguay was a buffer, and it took a couple of wars to prove it. No one had to go through Paraguay to get to anything worth anything. Thus, it became a question of whether it was better to look bigger on a map (possibly having to butt heads with Argentina over how to divide it), or to rape the country for what you can and then leave.
Versaille was a case of how to diminish a country that was still a threat (or would rebound to one in short time) coupled with how to get the most out of germany, while not creating a worse situation for Europe as a whole. Paraguay is almost completely different: a devastated country which won't be a military/economic threat, doesn't hold any strategic key and won't really change political/military reality for any of it's neighbors no matter what you do to it.