From my point of view, now buying a Van is just useless, firstly everyone would buy it for space and storage, now basically the trunk is the same size as the SUV or OffRoad class I believe, although i know, that Rockstar sucks placing the right cars in the right categories,
Secondly, its unique and that's why I bought it, talking about the uniqueness I would consider making prices a lil cheaper, or somehow improve the vans in general, Cuz throughout my playing, I've never seen a Van without mine on the road. Now nearly everyone just drives a Scout / Jugular / Kamacho / Rebla, just because it is nearly the best at everything. I would mainly focus on improving vans capacity, and making the suv's a lil smaller.
Trucks: 500Vol, leave at it is
Vans: 350-400Vol
Suvs: 100-150Vol
Maybe?