Technically, to paint a landscape you would use landscape layers to paint in the landscape tab, which is slightly different than painting by vertex. You still may often paint models by vertex with a pretty complex material setup. You can use your vertex colors as a blend in a lerp node between multiple colors, normals, or…
If you try to pump substance files themselves into an engine, you need to take care of a few things. Some nodes don't work outside of SD, and you can easily make a substance that is way too expensive to calculate in-engine. Unless you have a specific need for publishing substance files to your engine, definitely export…