Symbolism, Imagery, Allegory
Is there a better way to show that pretty much every character we meet is duplicitous than to deck them out in costumes and disguises? If there is, we don't know what it is. And indeed, this play is brimming with masks and disguises, reminding us time and again that nobody is up to any good. Just like they don't look like themselves, so, too, do they hide their real motives and, at times, even their true identities. Masks and costumes, then, symbolize the overall difficulty of telling who's honest in the play.
Sometimes disguises spell disaster for honest and upright characters, like when Antonio's virtuous wife is raped by a villain in disguise during a party. Other times, though, disguises end up helping upright characters, like when Vindice—in disguise—tests his sister and finds out that she's just as virtuous as he's thought.
As for the bad guys, disguises can be dangerous for them, too: The Duke is lured to his death by a skull disguised as a living woman, and Lussurioso is stabbed at a dance party under the cover of costumed entertainment.
Vindice's disguises are perhaps the most revealing, though, since they hint at the dishonesty this seemingly upright man engages in to gain revenge. The revenger bent on justice being disguised as a villain for much of the play shows the contradictions that a revenge obsession gets somebody into—he has to be bad to achieve what he sees as good. So much so that after a while, we start to wonder whether this really is a disguise, or perhaps Vindice is actually corrupted by his crusade for vengeance and becomes bad.
We definitely start to think he's turned to the dark side when Vindice, just done killing Lussurioso as part of a costumed entertainment, blames the death on a nobleman who had nothing to do with it. This pretty much seals the nobleman's coffin, meaning Vindice has just killed two men—of varying degrees of innocence when it comes to crimes against him—in a matter of moments. Looks like his bad guy disguises might fit him a little too well.