Campaigns contain only new mission files and campaign files. Mods are something that add, modify or remove assets such as models, textures, effects and tables. A campaign becomes a mod when it contains more than missions and campaign files, or replaces existing missions with its own versions.
If someone creates a campaign for a mod and if a campaign is packaged as a vp-file, the file can be copied into the mod's root directory. Or if the campaign is not packaged, it can be copied into the mod's \data\missions directory. Since the campaign does not modify the mod in any way, except add new missions, there is no risk in breaking rest of the mod. This applies to mediavps and vanilla FS2 as well.
Campaigns can also be packaged like mods, meaning they are installed like mods and have their own mod.ini. Which allows them to be selected from the launcher.
