It's actually Stokes at fault for the non-offside. You can see Vincelot waving at him to get up in line with him, Ricketts & Reda. Willis is caught as he knows he has his man on his shoulder but can't step up because he sees Stokes playing him onside.
Lameiras might have to take a bit of the blame as Vincelot was covering with Stokes and ends up in the central defence rather than in his normal position in front of them which would have closed up the acres of space Crewe had.
Granted, WIllis could have done better at dealing with the cross.
A good goal to concede though. TM will get right in there and sort it out and we can hopefully learn from the three mistakes: Switching off after scoring, not covering your full back & being aware of the defensive line at all times.