THIS!!!! Precisely why I don't think we're going to buy Vidal. We have too many No. 10s who are all very good and if we were to play Vidal and Herrera behind them then either the two box-to-box midfielders will see less of the ball, or the creative number 10 will and in each scenario you're not getting the best out of the individual player.
You can play two box to box players with a 10. It just means 1 up top rather than 4-4-2.
It's van Gaal's preferred formation, a 4-3-3 with a forward point as he's stated lots of times before.