Green’s Dictionary of Slang

(UK Und.) the victim of a crooked gambling game.

1591		Greene Second Part of Conny-Catching in Grosart (1881–3) X 83: The vincent, for that is the simple man that stands by & is not acquainted with their cosenage.
1608		Dekker Belman of London F3: I take six to one saies the Gripe, I lay it cryes the Vincent, and so they make a bet of six crownes, shillings, or pence, as the Vincent is of ability to lay.
1862		E. de la Bédollière Londres et les Anglais 318/2: vincent, [...] pigeon, celui qui est plumé au jeu.