I'll be the first to admit that Vidal hasn't played up to hit full potential, but the main difference between him and Marchisio is that Vidal has physical presence on the pitch even when he's not playing his best. As opposed to Marchisio in the past few seasons when he sort of slipped completely out of the picture when things where not going his way, and completely dominated in other games. No matter how many balls Vidal loses, he makes them up by sprinting 30m and taking the ball back or forcing an error, he is just relentless in that aspect of his game. To me, Vidal and his physical presence is one of the three main factors contributing to our excellent start, the other two being Vucinic and his movement and Pirlo with his fantastic vision.
I'd like to bring up the Viola game here, he technically he had a shitty game - threw away many balls and didn't make any really tough passes, but damnit if I didn't see him almost collapsing in the 70th minute by the left sideline, tackling the ball out and then hauling in 50meters to defend after the goal kick. Marchisio just doesn't have that, and that's fine - but that makes it so much easier to pick on Marchisio when he has an off day.
/rant