A gamester who does not immediately pay his losings, is said to vowel the winner, by repeating the vowels I. O. U. or perhaps from giving his note for the money according to the Irish form, where the acknowledgment of the debt is expressed by the letters I. O. U. which, the sum and name of the debtor being added, is deemed a sufficient security among gentlemen.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Vixen * UncleNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes