I don't see why round eliminated, then win rate (the current tiebreaker), then final vault size (the original tiebreaker) isn't sufficient. In the rare situation you need a tiebreaker after that, I'd say number of wins, and only after that should a rating system be necessary - preferably not RPI, there are far better ranking systems available and it's not really more objective than the events that actually occurred.
<iframe style="width:934px;height:371px;border:none;"src="https://docs.google.com/spreadsheets/d/1t3rAwDGtBSKKa1wflHGLJZpzil-swJvSYK1ziAzhe1w/pubchart?oid=472966616&format=interactive"></iframe>Click on an element to highlight its trend line.(Underworld is removed.)
(Now if anyone is fancy with Google sheets, they can make a pretty interactive graphic like AA did on page 4)