There was no captain named Vicksburg in the civil war, so no person of that name could have been important to the union or anybody else. Perhaps you are thinking of the battle of Vicksburg, which was critical.
Vicksburg is the most important Union victory.
Victory at Vicksburg secured control of the Mississippi Valley for the Union.
battle of vicksburg and gettysburg
They have burg at the end
Gettysburg; Vicksburg; Appomattox , Antietam ,Shiloh
US Naval commander Samuel P. Lee was the first Union commander to demand the surrender of Vicksburg Mississippi. This demand was made on May 18, 1862. Lee was the captain of the USS Oneida. Just prior to this the City of New Orleans was captured and Baton Rouge also fell. The Union navy then set its sights on Vicksburg.
The Battle of Vicksburg gave the Union control of the Mississippi River.
Vicksburg was so important because you had to control that city to control the Mississippi River, other wise no matter what else the union controlled, the Mississippi was worthless.
The Union could control the Mississippi River from Vicksburg
That would be the Siege of Vicksburg.
Vicksburg - U.S. Grant (Union) Gettysburg - George Meade (Union)
Northern success in the Siege of Vicksburg was important because the Union had a plan called the anaconda plan. The anaconda plan is when the Union planned to cut the Confederacy in half. By doing this, the Confederacy would not have any way to obtain (get) supplies (food, clothes, weaponry, etc.) When the siege of Vicksburg was a success, the Union was able to carry out the anaconda plan since the Mississippi River is in Vicksburg, and by taking over the Mississippi River, that split the Confederacy into two.