I know this is probably a stupid question but I'm (kinda) new to the world of Vocaloid so...

they are written by people who buy the vocaloid software, some of the very popular ones are made by actual musicians. That's what makes vocaloids so popular, you make the song, the fans have control. So if you really want to know who writes them the best you can do is find the author of some popular ones, like world is mine, that's made by a person named ryo. hope this helps!