Wow, who needs singers?
I was wondering about that vocal sound, then I read the explanation.
I'll have to look into the Synthesizer V.
Thanks, captain!

Interestingly enough, the version of
Synthesizer V I'm using is the free version, and it's a lot more functional (and better sounding) than the
Tiny Vocaloid editor I'd been using. While the
Tiny Vocaloid editor only allowed a single voice limited to 16 bars, the Basic version of
Synthesizer V allows for two voices and unlimited song length.
Plus, the Tiny version of the
Vocaloid editor is no longer available, so it'll cost around $200 USD to get into the game.

There's currently only one English
Synthesizer V singer available, but there are enough parameters to adjust that you can get a fairly wide set of voices from that single one. For my purposes (demoing a song), that's sufficient.