What are Vigors exactly? (spoilers likely)

Plasmids were thoroughly explained in bioshock but Vigors have barely an explanation. Maybe they go into it with a future dlc but what exactly are the Vigors, they aren't made of Adam and they aren't powered by Eve, it seems that the Vigors ARE salts as you can find multiple bottles of the same vigor and they just give you salt when you already have that vigor "installed" did I miss the explanation for Vigors like I did the explanation of the songbird? I know Fink mentions a brilliant biologist he's observing through a tear, but just like the Songbird, that's all the info we get.