I'd say very low. Stern reran Tron last year, except they didn't use the term "Vault Edition" then (when Avengers was being run). I think the only difference from the initial Tron release was the different translight.
Of interest is how quickly Stern turned on the run of Ironman. I expect Stern has already planned the next Vault Edition, and have the parts for it. But they probably hold off the build and only run them when a new model isn't selling as well as they expect. It makes economic sense. You wouldn't want a Vault Edition taking sales away from a new run.
...I'd say the health of the Pinball building industry is indicated by the number of Vault Editions!
