To pander to one's vices is to act as an agent to them, and such an agent is termed a pander, from Pandarus, who procures for Troilus the love and graces of Cressida. In Much Ado about Nothing it is said that Troilus was “the first employer of pandars” (v. 2). (Shakespeare: Troilus and Cressida; Chaucer: Troilus and Cresseide.)
“Let all pitiful goers-between be called to the world's end after my name, call them all `Pandars.' Let all constant men be `Troiluses,' all false women be `Cressids,' and all brokers-between, `Pandars.' Say, Amen.” —Troilus and Cressida, iii. 2.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Pander from Infoplease: