I wrote something on the French perception of the Indochina war (to call it the "First Vietnam War" is not common in French historiography, and could be said inaccurate as a lot of fighting was in places where the ethnic Vietnamese are a minority) here.

There was a relative indifference of the French population to this conflict, but the French Communist Party (PCF) and left-wing movements and intellectuals were quite involved in antiwar activism. To quote the 1952 op-ed of the March 6th, 1952 communist newspaper l'Humanité : "Félicitations au succès du Vietminh. Nous sommes de cœur avec lui. Nous envoyons aux troupes du Vietminh notre fraternel salut et notre témoignage de solidarité agissante." (Congratulations to the Vietminh success. We are with all our heart with them. We send to Vietminh troops our brotherly regards and tokens of acting solidarity). Sabotage was not unheard of in communist trade unions (remember that we are only a few years after the Résistance developped an entire art form of sabotage) : an example during the hard social conflict of 1947 (mostly unrelated with the war in Indochina) is the sabotage of the railway between Paris and Lille (which led to a train accident and 16 dead) : you can see here a very interesting newscast of the time. Later on, during the Algerian events, there would be a famous group of left wing French anticolonial militants that would engage in actively helping the FLN (called porteurs de valise, most famously with the Réseau Janson, defended in their trial by a renowned left-wing lawyer, R. Dumas).

There were strong rumors of sabotage among the troops, who deeply resented this : to quote an example, in the memories of Jacques Jauffret (a veteran of the 1er REC in Indochina in 1953-1955), titled Crabes et Alligators dans les Rizières :

"A mon arrivée en Indochine, en février 1953, mes camarades me mettent tout de suite au courant : il faut vérifier tout le matériel venant de France: des ouvriers politisés de nos usines d'armement ont pris l'habitude de saboter les armes et des munitions destinées aux combats contre des communistes, nos adversaires en Extrême-Orient.[...] Nous sommes d'autant plus furieux que le matériel américain nous parvient dans un parfait état. Il nous arrivait même souvent de découvrir dans une culasse de canon de char, une cartouche de cigarettes Chesterfield, placée là, à notre intention par les ouvriers des usines de Milwaukee. "

When I landed in Indochina, in Feb. 1953, my comrades put me in the know : one must check any ordnance or equipment coming from France : politically-minded workers in armaments factories took the habit to sabotage weapons and ammunitions to be sent to fight communists, such as in the Far East.[...] We are all the more furious that american supplies always come in impeccable condition. We even sometimes found in a tank cannon a carton of Chesterfield cigarettes left there for us by the Milwaukee workers".

Such sabotages and their intentional nature (could also have been that the French postwar industry was lacking in quality) have been accredited in historical literature^1 yet both remain a matter of debate in France (the topic is obviously politically laden).

In any case, the scene in Apocalypse Now is definitely realistic and the result of sound historical research as it depicts the resentment towards the Metropolitan French Left-Wing such colonist hold-outs would have felt, and the idea of sabotage would have been a rumor frequent in former veteran circles.

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^1 : Passés à l'ennemi. Des Rangs de l'Armée Française aux Maquis Viêt-Minh 1945-1954, Adila Bennedjaï-Zou, Joseph Confavreux, Tallandier