vietnamese.ldf's \ProvidesLanguage

[Rimole]

There are, for example
bahasam.ldf 2017/02/15 v1.0m Bahasa Malaysia support from the babel system
british.ldf 2017/06/06 v3.3r English support from the babel system****
french.ldf 2023-12-19 v3.5r French support from the babel system
latin.ldf 2021-06-27 v4.0 Latin support from the babel system
naustrian.ldf 2024/01/19 v2.14 German support for babel (post-1996 orthography)
ngerman.ldf 2024/01/19 v2.14 German support for babel (post-1996 orthography)
russianb.ldf 2021/01/10 v1.3m Russian support for the Babel system

i.e. bahasam.ldf has

\ProvidesLanguage{bahasam}
       [2017/02/15 v1.0m Bahasa Malaysia support from the babel system]

but vietnamese.ldf just has

\ProvidesLanguage{vietnamese}

i.e. the [...] is missing.

(And the file gives Werner Lemberg as current maintainer, while https://www.ctan.org/pkg/babel-vietnamese says "(inactive)".)