The thing with fielding two box-to-box midfielders in a midfield 2 is that, regardless of what sort of defenders you have, they will be exposed often every match. Even when we played with Scholes and Keane, we would concede a number of chances against opposition of good quality and leave our defenders exposed against them only for Schmeichel and our defenders to perform heroics or concede 1-2 goals. Such 2-man all-action midfields are great against opposition with poor attacking setups, teams that push everyone high up, or teams that don't look to attack at all. However, such midfields will leave us exposed against opposition teams with good attacks.
If one has to go with a 2-man, all-action midfield, with a 4-man defence setup, then one has to go with a "you score 2, we'll score 3" mentality, which doesn't always work against world class opposition. However, if van Gaal goes with a 3-man-defence setup, then Vidal-Herrera is reasonable given that there's an extra defender protecting them through the middle. However, if van Gaal goes for a 4-man-defence setup, then Vidal won't be the best option, and we'd be better off with someone with more discipline in their game yet able to do the damage at the other end (Schneiderlin, Neustadter, Badelj, Kramer, etc.).