Vocaloid's costume question?

It's decorative, and is usually described as a belt or strap. As far as I know it started with Miku and I think is pretty much unique to the Crypton Vocaloids. If it did indeed start with Miku, then it would have been conceived by the artist KEI, but if he's ever said what it is supposed to be, then that would be news to me.

I did recently see somebody refer to it as a "suspender" which is an interesting interpretation. It does look like an elastic trouser suspender hanging loose instead of being worn over the shoulder.

Looks like an ornament to me. Personally I'd say artists add that kind of thing to add variety to a scene or character since Manga style is pretty simple at its core. For example: if you take off her clothes, what's left? Just an outline of her body plus a few details and basic shading to give a sense of depth.

Having struggled to comprehend why certain drawings are better than others, variety is one of the answers I've found. In the case of Manga and Anime characters, variety usually is achieved by giving a character unique hair and clothing.

A belt or strap.