My only issue with 'Vengeance' is that it's just a wild goose chase of cat and mouse, no real action in it really when compared to the others.
If any of you guys feel a lack of cohesion in the sequels, that's because all of them other than 'Live Free Or Die Hard' are based on either a novel or another story. None of these films other than LFODH came from a script written specifically to be a Die Hard film.
'Vengeance' completely changes after the first hour because it was originally a film called 'Simon Says' which was to be the next Lethal Weapon and the second hour of the film was re-written when it became Die Hard III and the first half is virtually untouched from its original script.