"Vipi" also means "how", so "mambo vipi" is "matters (as is your affairs) how" basically meaning "how are things?" I've never seen -pi to mean which in this way in conversational Swahili.
So, if vipi is for vi- nouns, why do we say mambo vipi? Isn't mambo a ma- noun?
i think "vi-" prefix is also used more generally when the noun isn't known, or when agreeing with a verb that has no noun class. sort of like how most unknown objects are treated as N-class for possessive pronouns.