Green’s Dictionary of Slang

of a losing gamester, to pay off his debts with an IOU.

1709		R. Steele Tatler No. 12: Pox on it! don’t talk to me, I am VoweI’d by the Count, and cursedly out of Humour.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: 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.
1801		‘Modern Dict.’ in Sporting Mag. May XVIII 102/2: [as cit. 1788].
1811		Lex. Balatronicum [as cit. 1788].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum.
1864, 1867, 1870		Hotten Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1895		Crowe Aus. Sl. Dict. 92: Vowel,‘to vowel a debt;’ to acknowledge with an I.O.U.