Even after a defeat at Vicksburg, Grant launched another attack because

Vicksburg sat on the Mississippi River, so capturing it would give the Union control of the river and divide the Confederacy. That strategic advantage mattered to Union victory over the long run, which is why Grant continued the campaign despite setbacks. The correct choice is “Vicksburg was vital to a Union victory in the long run.”

AI-written and checked against the verified answer.