Wouldn't that have to be done before the French colonized the area?
Yeah any intervention of China into Indochina late in the Qing dynasty means war with France. This actually happened in the French-Sino war of 1884-1885 where France took control of, yes, North Vietnam.
They had their eyes on the region since the 18th century, good trade, good missionary work. They ain't going to let that go to waste by Chinese.