Dermot Gallagher's explanation of Villa's disallowed goal and VAR below. Basically as soon as the ref believed that Grealish dived he has to make the decision there and then, rather than allow play to continue and refer back to the incident, as VAR cannot be used to make a decision on the dive. VAR would have only checked for an offside or foul by a Villa player in the build-up to the goal, therefore had Grealish been guilty of diving, VAR wouldn't have disallowed the goal.
Interesting - what's the actual rule for diving if play continues? 9 times out of 10 when you see a dive it's when the ball goes out of play for a goal kick, so it's not something I'd ever really considered much before. But I don't see why the ref needs to blow the whistle as soon as he thinks it's a dive? If the player who he thinks has dived doesn't impede anyone, why can't he just let play continue and then book the player that dived when play stops naturally? Like they do when they play advantage for a foul.