A child born to US citizens in a foreign country or US territory is a natural born citizen. Jus soli is as though the child was born on US soil.
That’s a matter of settled law. A child is NOT a natural born US citizen if he was born to foreign parents abroad. He would be a US natural born citizen if born here regardless of the parent’s citizenship status.
There are some exception like diplomats, whose children born here receive the citizenship of their country, not the US.
Some countries may extend citizenship to any one born on their soil along with whatever citizenship was claimed by their parents at the time of their birth.
So, all the children born to US Servicemen in Vietnam are considered US natural born citizens?
The Act also establishes the United States citizenship of certain children of citizens, born abroad, without the need for naturalization: "the children of citizens of the United States that may be born beyond Sea, or out of the limits of the United States, shall be considered as natural born Citizens".
Please quote your "settled" law.