I don't have or know anything about VB Zone Converter, sorry.
Looks like I had an old, crappy tool for exporting .s3d-based zones to .obj:
link. Unity and any 3D modelling programs should support .obj format. However, it would certainly not capture animated textures or vertex-animated geometry (waving flags, etc). Can't remember if the tool exports static objects (like trees, tents, etc), but I think it does.