The population of Vietnam was 42 million in 1970. Serious question, is there a huge spike in mixed Caucasian population from the war? One million+ rapes is close to 1 in 5 women (<20 million women, less than 1/3 of that teen or older).
It is impossible to even guess the number of rapes that occurred during the Vietnam war much less get a number that would be considered credible. It's not like the Vietnamese women who were just raped by US Army soldiers are gonna walk up to the US Army base and report it.