I'm not typically a fan of Vocaloid wallpapers, but that particular scan + what was accomplished in this collab makes this very eye-catching and lovely.
The background compliments Miku very well, and gives the entire wallpaper a feeling of movement and a nice load of vibrancy; which are always nice features I enjoy seeing. I especially like the effect of the slight light textures on the stars.
The main criticism I can offer is the quality of one of the textures (or it could be because of something else?) looks like it had some JPEG artifacts and/or was enlarged, so it looks less crisp in full-view; it's really noticeable in Miku's hair and the typography. Other than that crit, this is a fine wallpaper. Great work. :)