If your players are not motivated by your Captain, his Influence is too low. In which case there is only one solution: change captain. Vidic has an Influence of 12 or 13 at the start of the game, so is never going to be a great captain. When I took over United I made Ferdinand the captain, despite him being injured at the time, his influence is 17. Giggs, Rooney and Fletch also have decent influence. Fletcher is a fair choice of those three, and his influence will go up later too (or did in my game at least). A robust defender or otherwise dependable player you will be likely playing in almost every game you can is the best choice of captain.
It's best not to change captains mid season, do it at the start. When I took the captaincy from Vidic he wasn't happy at all but after some stern words and the threat of time in reserves he came around. I also made sure I was already at the point where he "felt he had a good relationship with me" BEFORE i demoted him, which you can do be praising good player or getting him to tutor someone.
For general Motivational issues, make sure you have a squad with decent Determination and keep morale high. Be demanding against "easier" teams. You EXPECT a win and, if they are not leading at half time, be disappointed or even angry. Single out players at HT in the team talk if they're playing poorly and have words after the game in private chat. Oddly, if you tell a player he was **** and must do better, they almost always respect you for it and say they'll do better next time. That said, you should be more forgiving for newer or younger players, be encouraging instead. Tell them you "have faith in thier ability."