What does 39 mean in terms of vocaloid?

There's no established meaning for "39" in the Vocaloid community. It's possible you're thinking of something else:

* 39 (ミク) - A Vocaloid song: This is a popular song by Mitchie M featuring Hatsune Miku. The lyrics are about the feeling of loneliness and isolation.

* 39 (39 or サンキュー): This is sometimes used as a slang term for "thank you" in Japanese. It's possible you're seeing it in the context of fan appreciation or messages of thanks to the Vocaloid community.

If you can give me more context about where you saw "39" in relation to Vocaloid, I can help you understand it better.