The United States said Thursday that “substantial progress” during negotiations in Vienna to save the Iran nuclear deal had been made, deeming an agreement possible within days if Iran “shows seriousness” on the matter. The Vienna talks, which involve Iran as well as Britain, China, France, Germany and Russia directly, and the United States indirectly, resumed in late November with the aim of restoring the 2015 deal. That accord had offered Tehran sanctions relief in exchange for curbs on its nuclear program, but the United States unilaterally withdrew in 2018 under former president Donald Tru…