Re: Listing as an Alien Trader #poland
Vietnam was known as Indochina, so it makes sense that in 1921 he may have listed China as his father's place of birth. Perhaps he told the enumerator Indochina, but it got written as just China.
"When did Indochina become Vietnam?
French Indochina was formed on 17 October 1887 from Annam, Tonkin, Cochinchina (which together form modern Vietnam) and the Kingdom of Cambodia; Laos was added after the Franco-Siamese War in 1893. The federation lasted until 21 July 1954."
And, there is a wiki page:
History of the Jews in Vietnam
A genetic contribution from the Far East into Ashkenazi Jews via the ancient Silk Road https://www.nature.com/articles/srep08377
Good luck in your search,