I'm not able to verify the reference, but I found this quote on a Postal History blog of Canadian slogan cancels:
Quote:
According to H.E. Guertin, the "V" for Victory" slogan was first conceived by a Belgian lawyer Victor De Lavelye in January 1941 while in London, England, and its use spread to many Allied nations.
(The Wartime Mail and Stamps Canada 1939-46)