The thing about Lumi being done in one take may have been hearsay? But Vocawiki also says her recordings were done entirely for Vocaloid and not borrowed from any other sources, which made me realize that that might not necessarily mean borrowing samples from other voicebanks (though I'm sure that's happened) as much as borrowing samples from other recordings of the voice provider?
For example, in Avanna's case, her VP is a professional singer and I wouldn't be surprised if they took bits from past instances of her singing. In Miku's case many say her updates use samples from past versions as well.