Enemies with mundane armour superior to Victor's special clothes show up. They still die in droves.
The armour is literally made of cardboard.
The armour is directly harmful to its wearer.
Armour that withstands powerful attacks inexplicably crumples under less powerful ones. Even with technical explanations, there's simply no way to realistically account for the inconsistency.
Armour is useless because the authors simply can't seem to make up their minds about it.
Plasma weapons pierce kevlar vests and high strength polymers easily. But Mag Shield (for Magnetic Shield) armor generate a strong magnetic field that easily deflects plasma. Too bad Mag Shield armor is heavy and delicate, and as such doesn't help much against ballistic weapons, and then you have Power Armor, combining high strength polymers with a Mag Shield system, weighting 300 kilograms a suit but using servomotors to allow the wearer to use it without Super Strength, effective against both ballistic and plasma, but requiring regular refueling or else it becomes a paperweight that costs a few million dollars... In short, different kind of armors have different strength and weaknesses. Someone wearing armor unadapted to the situation might as well be naked. Someone wearing armor fitting the situation they are in enjoy decent (but not perfect) protection.
Averted: Armour isn't useless - but it's not impenetrable either.
Enforced: "This is an action movie, not real life. You know how boring it would be every time our hero has to unload a shitload of bullets onto one guy?"
Lampshaded: The Hero said: "Is that cardboard, or what?"
Invoked: Emperor Evulz realises this trope is in action and organizes a mass production of armour-piercing rounds for his Mooks to use.
Exploited: "I'm not bringing that armor. Knowing the fight we're in, it's not going to help."
Defied: The top quality weapon manufacturers are called in to make sure armour won't be useless.
Discussed: The Hero said to his Sidekick: "Don't worry about aiming for joints or silly crap like that, Evulz troops are cheaply outfitted, their powered-armor, while massive, has all the integrity of an egg carton."
Conversed: Two guys talk about storming the castle. One says: "Look, I watch TV, I know how it works. There'll be five hundred Mooks in there armoured to the hilt, fighting against us two with no armour whatsoever. We're bound to win."
Because their armour is so weak, the army loses to its better-equipped enemies and they lose the war with high casualties. It goes down in history as the worst military defeat of the century.
Armour is useless because the manufacturer of bulletproof vests was bribed by Emperor Evulz.
...But his own troops' armor is just as rubbish.
R&D shifts away from making tougher armour to making stuff that increases mobility and stealth; since armour won't stop a good hit, might as well ensure that hit never connects.
Played For Laughs:
Everyone sees being ordered to wear armour as a punishment.
An armoured character falls over and every scrap of armour on him collapses into dust.
Played For Drama:
Hero's friends die tragically due to uselessness of their armour.
The massacre of armoured troops is compared with the grief of their families as they watch helplessly from afar, as well as their disbelief and outrage as the unarmoured Enemy racks up a body count.