Had a quick look online. Some say it's next to the power steering reservoir or by the battery under the passenger seat.
I'm sure someone who has a Vito will let you know.
MB Vito 111CDi / W203 C Class Saloon 320CDi Avantgarde
I'd Try looking at the Drivers Side Bulk Head, to the left of the Large Fuse box and almost above the EGR Valve. Its help on by 2x10mm Nuts with securing washers. Beware of the Earth 10mm Cable nut, spray liberally with WD40 before trying to undo.. ask me how I know..