Manchurian ruler is the Qing, similar to the Mongolian as Yuan.
I guess Tu Duc sent envoy to pay tribute and to seek help from Qing in the fight against the French. I am trying to understand why the Qing failed to protect Vietnam despite the fact the Nguyen had formally recognised the Qing as suzerain. Yes, we paid tributes, but we received obviously no protection in return. When Chinese were willing to intervene, that was too late.
Bui Vien in 1876 did what you think. He asked USA for help to counter France. But envoy 1888 is meaning less, because agreement Quy Mui 1883 .Vietnam was already under control (protectorate) of France .