Porsche has recently said its current focus is WEC not F1. A transition of a VAG brand to to F1 would take time and money. Money VAG have, time I think is in shorter supply.
Perhaps if there is some lessons learnt in WEC that translates across to F1, then another marque like Audi may move into F1, but I seriously doubt a 2014/15 start would be possible.
As to the Bugatti, Ferrari, Lamborghini et al table mentioned before, a little context sometimes changes perspective:
Fiat owns Ferrari & Maserati & Afla Romeo- All old F1 names however Fiat would never run against Ferrari in F1
VAG owns Audi, Bentley, Bugatti, Lamborghini, Porsche, SEAT, Škoda & VW as well as Ducati and a 20% odd stake in Suzuki.
The Veyron was a "loss leading technical exercise" for VAG in that they lost money on every car they sold, but gained technical knowledge and prestige that was worth the spend.
So for VAG, it comes down to which brand has the most to gain from a foray into F1??
Porsche has a solid background and history and sportcars and prototyes like WEC is a good fit.
VW is moving into WRC and this fits with the small car product that VW produces
Bentley has done some moderately successful racing recently but it isn;t really the target demographic for the plush millionaire / footballers wives / rapper-gangsta set that drive them
Lamborghini maybe especially seeing that recent moves in carbon construction with Boeing for the Aventador and Gallardo etc
Seat & Skoda...enough said
Audi would seem to have the right demographic and model line up to possibly benefit from an F1 role. Couple that with the past history of the brand as Auto Union and it would seem to be the best fit.