Vice City does not use files of type .wmb, as in 3DGS, they are more like normal models, which load faster. If you look at certain places in Vice City you will notice that the texturing is not done like a .wmb handles the texturing, ie. some places they are stretched and not repeating. They also use very low res models and textures, which also load faster.
An idea would be to try using models and for each model you create an invisible .wmb for the collision detection. Then try using the built in LOD system to get them to load when user gets closer, but then its not easy for the smaller entities, ie. units and cars, to load. Maybe you could add them into the .wmb that makes up the collision. Just an idea, never tried, so no proof if it could work.
I am currently working on getting a Vice City sized level to work. I use a huge terrain object and from it I calculate a height map, store it in an array, then I use another array to place different objects on the map, ie. grass, trees, rocks, buildings... Then I calculate where the unit is on my huge terrain and render the grass and trees and so on which are visible to the player.
