Geog~~~~~~~V.I. stand for what?

I am not quite sure about the answer...... But I think it is:

Virtual Isohypse.

I have heard of VE : Vertical Exaggeration for topographical map, but not V.I..

Why do I think V.I. = Virtual Isohypse?

Because:
1. You have mentioned that V.I. of the map is 20m... In this case, it is related to contour lines of topographical map.
2. Another name, that is similar to contour line, is Isophyse: which is a line that have a same elevation in topo maps...
3. V. either means vertical or virtual... Because this kind of line is imaginary and is used to represent the elevation of land on maps.

In conclusion, based on the combination from the above reasons, my answer for V.I. is Virtual Isohypse