If you have a vector file you can extrude the lines in most modelling programs. Blender included. It gives you really crappy geometry though and you may have difficulty capping things (like for buildings). You can also use a Boolean operation to then take those lines and cut a mesh (like one generated from a height map) into separate sections and have a particle system populate it with trees, or whatever. This is fairly advanced stuff though, it's not exactly simple. For things like buildings I'd suggest just tracing it out with vertices in the modelling program instead of importing the vector file, and then if extrude from those you'll get nice clean models that will be easy to cap and work with later on.