Almost every writing system was imported from somewhere else, including something like half a dozen evolutions of the one we're using now (which was Latin, which was Greek, before that Phoenician, before that Egyptian).
What matters is that the Vietnamese use the script to write their own language, which is not the case for (say) romanized Chinese.
So you're saying that because the country was once colonized, their writing system is not "real" enough for you, and you only consider it a transliteration? That seems extremely disrespectful.
That is a really dumb point. Then Finnish has no writing system either, because it was created by a swede in the 16th century. Strange how there exist languages without writing systems, yet people write them?
it's the only official writing system that we have. The non latin scripts have practically disappeared from modern life.
We had centuries of Chinese scripts, which is definitely not native, then a short lived Chinese-like writing system that is the closest thing to "native", (it's not, see "Chinese-like"). Even that was not used as official system for as long as the current latin alphabet.
What matters is that the Vietnamese use the script to write their own language, which is not the case for (say) romanized Chinese.