Rio - poor concentration (at times truly abysmal), weak in the air (just doesn't like heading the ball), strength ok but nothing special. Plenty of strengths mean he has been a very good player for United and for a spell was a superb performer.
Vidic - very slow (relative) and limited technical skills (not a pure footballer, for instance you couldn't imagine him in midfield). Obviously good at just about everything else.
McGrath - genuinely quick (fastest player at the club at the time), strongest player at the club at the time, most skillfull player at the club (I'm being totally serious. McGrath's flicked clearances were Cantona-like. He could dribble his way out of trouble. He played for Rep of Ireland in midfield!). The only United player in his league since George Best is Ronaldo. The likes of Keane, Robson, Cantona, Schmeichel, Rio, Giggs et al do not come close to McGrath at his peak for United. He was just embarrassingly good. We defended a very high line up the pitch then let quick forwards get through on goal (under Atkinson fullbacks were attackers not defenders so there were big holes in the defence) only for McGrath to accelerate back to catch the forward then dispossess him (not tackle); this happened so many times at Old Trafford it was ridiculous as you knew exactly what was going to happen.
Most of McGrath's acclaim comes from his time at Villa and his displays for Ireland in the World Cups. But by this time his knees were in terrible shape and the drinking was completely out of control. His performances for United were significantly better than those that earned him the PFA player of the year much later in his career. Watching him play for Villa, what used to strike me was how ordinary he looked compared to his United displays. But the clips of him against Italy in the 1994 WC give a clue as to how good he was when his body was in better shape and he had his incredible pace (he'd left United 5 years prior to this).
YouTube - Paul McGrath v Italy 1994
Hopefully neither Rio nor Vidic have McGrath's drink problems, or his knees.