Escalus seems like the only character with his head on straight in this play, don't you think? When Angelo sentences Claudio to death, Escalus points out that Angelo is being way too harsh. As the Duke tells us in the opening lines of the play, Escalus's legal expertise and knowledge of the Viennese people is unmatched (1.1.1).
Here's yet another possibility. We could say that when Duke Vincentio dons a friar's get-up and runs around taking confessions, he acts as a kind of guide or mentor. If, on the other hand, we're feeling skeptical of the Duke's behavior, we could just say that the guy has a serious "God complex." Why else would he pretend to go on vacation and then stick around to spy on everyone?