Vaguely describes action that is unclear. If you vaguely recall meeting someone once before, you barely remember him or her, what you talked about, or even where you met.
The adverb vaguely probably comes from the Latin word vagus, which means “wandering,” or “uncertain.” Its origin is unclear, which is fitting for a word that means "mistily." If you don't want to go to your friend's tuba recital, you can say vaguely, "I think I might have something else to do that night. But if I don't, I'll try to make it."