"In The Vague" is an expression that means "i kinda know but not really." It means that you aren't sure about something.
Ted: "Hey, could you help me with my calculus homework"
Mike: "what is it on?"
Mike: "i don't know man, i haven't done those since freshman year, i am kinda in the vague about how to do them"
Ted: "aight man, it's cool"