So in a previous run I just triggered it by getting big enough to be an eligible rival for Ming, in which case they will break tributary as soon as you pop the mission (but you get the reward of +1 diplo stat for your ruler so that's cool). This time I actually wanna become a Ming tributary (so I can vassalize Dai Viet) but it doesn't actually seem to be feasible because of the trust requirement. The wiki states that just 100 opinion is required (and I tested - it's not enough) which seems sensible enough to me. How can you even get 15 extra trust from Ming when you can't become a tributary otherwise, and they're guaranteed to rival you before you get strong enough to ally them?