You shouldn't have stopped the Germans in Bose. You need that huge force in Hanoi to disperse and the best place for it to disperse to is north into your territory. You want the Germans to mucking about all around the interior so they suck troops north away from Hanoi, Haiphong, and Hainan. As long as you don't let them take Luang Prabang or Qinzhou, (which they won't) all you have to do is wait until Hanoi or Haiphong is weak enough and every German soldier north of that line is knocked out of supply and easily disposed of. You can't do that in Vietnam as they will be supplied from the coast every step of the way and you'll just end up pushing them back until there are 15 German tanks sitting in Saigon with no way to dislodge them.
With Qinzhou and Luang Prabang, you've made the perfect noose now all you have to do is let the Germans stick their heads in it.