Hi Cecilia, there are three main pieces:
1) a bobbin (it is the barbell shaped piece that holds the thread)
2) a shuttle which holds the bobbin inside it (it is a bullet shape)
3) a carrier which holds the shuttle (this piece is attached to the machine and can be seen when the slide plate is open)
"Bobbin Case" is not a term used with VS machines, it is the shuttle that holds the bobbin for VS machine. Did you see the pictures on Wikipedia? That site shows all three pieces clearly so hopefully you can communicate effectively with the seller.