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The oldest Vocaloid song ever released is 'That Wonderful Love Once More' (Ano Subarashii Ai wo Mou Ichido) sung by Kaito and Meiko on July 24th, 2003 (BEFORE Leon and Lola were even released). A fanmade PV of it could be seen here (It also includes the facts about Crypton and Kaito and Meiko when they were in development)

That time Crypton haven't decided on Kaito and Meiko's name, and so the title of the singers are 'Prototype Vocaloid' (So this song is created before Meiko even has a name).

That Wonderful Love Once More is the oldest non-demo vocaloid song ever to exist.