Although not about Vietnam per se, Ricard Fogarty's Race and War in France: Colonial Subjects in the French Army, 1914-1918 contextualizes tirailleurs indochinois along with their African counterparts. Kimloan Vu-Hill's Coolies Into Rebels: Impact of World War I on French Indochina examines the impact of wartime service upon Vietnamese laborers and soldiers abroad.

When it comes to the soldiers from Indochina then I am going to have to be clear: there is little to no English sources on this subject. There are some brief mentions in more general works of Vietnamese history but are not nearly enough to cover the subject. The only secondary source that actually takes time to tackle the subject on an academic level is Race, Empire and First World War Writing (Chapter 2 - Sacrifices, sex, race: Vietnamese experiences in the First World War) published by Cambridge University Press. There are plenty of French sources on it, but if you're like me and can't read French, then that makes life a little harder.

Here are some suggestions on French sources:

Lettres d'Annamites - Lettres de Guerre, Lettres de Paix by Jean Marquet. A compilation of letters written by a Vietnamese soldier and released shortly after the war.

Les Troupes Coloniales dans la Grande Guerre by Claude Carlier and Guy Pedroncini (ed.) Contains a chapter on the Indochinese participation.