Either write:

L’indépendance du Viêt Nam a été officielle en 1954.

or:

Le Viêt Nam est devenu indépendant en 1954.

I would suggest:

L’indépendance du Viêt Nam a été déclarée en 1954.

I’m not exactly sure why, but I would try to work the French equivalent of “recognize” (reconnaître) into the sentence, preferably in the active voice:

“In 1954, France officially recognized, once and for all, the independence of Vietnam” (I added the “once and for all” to address the fact that their independence had been “officially recognized” by France once before in 1949, but you probably can omit it):
So in French it might look like this:

“En 1954, la France a officiellement reconnu (, une fois pour toutes,)
l’indépendance du Vietnam.”

If you want to use the passive voice to get “Vietnam” up front (and without the “once and for all” crap):

“Vietnam’s independence was officially recognized by France in 1954” which might look like this in French:

“l’indépendance du Vietnam a été (était?) officiellement reconnue par la France
en 1954”

If you want to speak more of recognition of the independence by the international community and not just by France, you could replace “France”/”la France” with “The Geneva Conference”/”La Conférence de Genève’ in my two suggestions (and in the “passive voice” version you might want to replace “by”/”par” with “during”/”pendant”.

(NB: Any French speakers who know better than I do whether “officiellement” should come after “reconnu/e” instead of before it, and also whether the “e” in “reconnue’ is/is not correct are invited to make these corrections and any others that I have surely made.)

Complementary:

L’indépendance du Viêt Nam a été officialisée en 1954.

i.e. formalized.