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!