It's true that they had a colony in New Guinea since the 1880's pr so, but that doesn't really count for southeast asian experience... firstly, New Guinea was a different climate (highlands, jungles) secondly it was populated by tribes who didn't have anything in the way of modern technology, communications or organization, and were thus easy to govern, and thirdly New Guinea was very remote and had IIRC less than a hundred or so full time German administrators, plus plantation settlements in some coastal areas.
Not comparable to Indochina, where they would have to deal with large societies who have had ample exposure to the modern world and where communications were a great deal easier, which works to your disadvantage if you're a small group trying to assert your domination over a couple million people. I don't see the Germans sending army corps to Vietnam to support their administrators there, at least not in the immediate postwar years when eastern Europe needed large garrisons to establish German rule. So my guess is that the Germans there were originally (1917-1922) ordered by Berlin to tread lightly and not upset things too much, and only then would Berlin start to show more interest in the exploitation of the new crown jewel among their colonies. That, in turn, can mean anything from iron fisted subjugation and aggressive de-Frenchfication to a sleazy economic penetration that leaves the French locals and the traditional local elites in power but sees to it that they do their business with Krupp, Siemens, IG Farben and the other cartels. Either way it can mean lots of things, both good and bad for Indochina.
And if the Germans show some restraint, they might actually learn a thing or two about cultural tolerance from all that exposure to foreign cultures. The prevailing views of Asians and non-Europeans in general was hopelessly naive and stereotypical back then... just like in most Euro societies. Instead of being a place where machoist young officers go and teach the natives proper respect for white men, Germany's new colonies could in the 1920's become places where young progressives go, to put new educational and economic ideas into practise which they can't try in the stiflingly conservative atmosphere back in Germany. (Pestalozzi teaching the Khmer? Bauhaus in Saigon?
) It could also become a place where war veterans unable to adapt to a peacetime society get sent on garrison duty, and where Fritz Haber's newest poison gases are tested on the rebellious locals who don't want to pay the head tax.
I could see IG Farben developing Agent Orange 50 years early. Maybe Hitler even spent a few years there, introducing efficient "prisoner clearing camps" in the Laotian highlands.