I suspect it's more that they must be into diminishing returns from the promotional value of winning Le Mans over and over again (last year excepted, of course). A VW Group entry into F1 would surely be run under the Audi banner to go head to head with Mercedes, and would time nicely with the R15's likely retirement from sports car racing in a couple of years.
Still think they would be better off buying Sauber. Audi's motorsport programs have traditionally been run in-house, and they would have an easier time integrating VWG management into Sauber than the traditionally-independently-run Williams. Honda, Renault, BMW and Toyota all found that Frank and Patrick do what Frank and Patrick want, for better or for worse. But buying BMW's old racing team might be too bitter a pill for Volkswagen...