Hey H, know the problem.  All the standard van ply lining (for work vans etc) don't bother with the roof, they only tend to do the floor and side walls.  There's a place I saw online think called SJH Joinery who do beautiful wood lining, including the ceiling but looks like it's just for VWs altho they might have some suggestions.  I haven't done this myself so this is just a suggestion but I'd see if:

-is there an existing roof lining you can remove and use as a template like the panels on the back doors or something (or is it just straight to the shell).

- if not, get some large sheets of cardboard (fold down a box or the like) and use them to work up a template for the roof in sections.  I'd go across the roof (rather than lengthways) as it will be easier to bend plywood this way (assuming the vito has a slight curve to the roof) and make the sections lighter for lifting. 

- work out how many panels you will need based on standard plywood widths if possible (so there is less cutting involved) and where it is going to be easy to screw into the shell of the van.

- get some thin, flexible plywood and use the templates to cut to shape with a jigsaw.  You should be able to get 2.7mm or 3mm.

- screw into place being careful not to go through the shell. 

You could speak to a local joiner.  They might not work on vans very often but they should be able to do it.   Or somewhere in between, they make the panel, you fit it or vice versa.  Be cool to hear how you get on. R