The US's contracts would have been fulfilled first regardless (albeit more slowly) without the DPA, because (as I said) the US was the first and paid the most. We know this because the US's Pfizer contract explicitly prohibited the company from exporting US-produced vaccine doses until March 31, but the company only began to export US-produced doses at the end of April because it still had to fulfill said US contract first (<https://www.barrons.com/articles/pfizer-to-export-u-s-made-v...>).
No. The US did invoke the Defense Production Act for certain medical equipment, to further ensure that its vaccine contracts are fulfilled first (<https://www.bloomberg.com/news/articles/2021-03-24/biden-use...>). (But, let me quote for others' benefit (<https://www.ft.com/content/82fa8fb4-a867-4005-b6c2-a79969139...>): "The DPA does not allow the administration to block exports overseas".)
The US's contracts would have been fulfilled first regardless (albeit more slowly) without the DPA, because (as I said) the US was the first and paid the most. We know this because the US's Pfizer contract explicitly prohibited the company from exporting US-produced vaccine doses until March 31, but the company only began to export US-produced doses at the end of April because it still had to fulfill said US contract first (<https://www.barrons.com/articles/pfizer-to-export-u-s-made-v...>).