So it seems you have got a little confused. While there are huge differences between spongeAPI, SpongeCommon, SpongeVanilla and SpongeForge. Sponge Forge contains SpongeAPI and SpongeCommon for all your plugin needs.
If you want to know more:
First of, head over to the sponge docs, they will tell you everything you could want to know.
SpongeAPI is a layer of code that most plugins solely interact with, SpongeAPI cannot run on its own (hence the API part → stands for Application Programming Interface).
This means that anyone programming a Sponge server will need to include SpongeAPI to make it able to run Sponge Plugins. So you have SpongeForge, SpongeVannila and LanternPowered all using the SpongeAPI.
SpongeCommon (I know you didnt ask for - but its nice to know) is where the majority of where the code for getting SpongeAPI to interact with Minecrafts vanilla code lives, so you will find this in both SpongeVanilla and SpongeForge however not lantern as that doesnt have the Minecraft Vanilla codebase.
SpongeForge is a extension to SpongeCommon, adding all the code to boot as a Forge mod as well as allowing SpongeAPI to get the Forge mods blocks, entities, etc.
SpongeVanilla is like SpongeForge only instead of adding all the Forge specific things, it just adds the code to boot