if this question is about warring reputation in the past.
answer might be: they did no more than Robinson Crusoe. A loner country.
No it was worse. It was like suicidal tendency. Because there is nothing to lose. After three centuries constantly in civil wars, discovering and assimilate new farm lands in the south, introduction of guns, learning alienated languages and culture forced into our throat by whatever imperialists or colonialists, it would be so easy for Vietnamese to be fooled by “too good to be true” communism idealistic. It was simply no effort left to care about somebody talking nonsense, or rationalize the words, or even be curious about where the f the origin of those words came from.
Just politely accept it and move on to adapt.
yes some said it is karma due to the atrocity we did to the Champa empire. Or a curse thanks to be specially skillful at killing neighbors. Vietnamese have already got used to live in villages, have all kinds of knowledge about their neighbors. One is not necessary liking his/her neighbor, normally he/she still gets to bear with it, because it is their village, where he/she can go as in Vietnam, every region is small and different. (The physical distance helps extending living culture perhaps) (Hanoi is travel-wise at least double closer to Hong Kong than travelling to Ho Chi Minh city, as a side fact). Such knowledge improved the efficiency and gave impression about skillful soldiers.
but no, they were not. Dead count should roughly dismiss the claim.