I highly recommend reading Castel's "Vicksburg: Myths and Realities" in North and South, Vol. 6 No. 7. As he concludes:
"Yet in some ways Davis' determination to hold Vicksburg benefited the Confederate cause. The reason is that it was
matched by a Federal fixation on taking it that ultimately led to almost 100,000 Union troops being used for this purpose, counting Banks' army at Port Hudson, and to Grant spending nearly half-a-year vainly endeavoring to get at Vicksburg from the north on land, through swamps, and even by digging a canal in hope of changing the course of the Mississippi, before he finally reached it by marching most of his army down the west bank of the river and then crossing it over to the other side below the fortress in transports supported by Porter's gunboats.26
Also, as we have seen, Lincoln's decision to pursue political and diplomatic objectives instead of military ones after the fall of Vicksburg benefited the Confederates by delaying Northern victory in the war and, on the Red River, actually jeopardizing it. This is why Grant biographer Brooks Simpson is correct in concluding that the most important outcome of Vicksburg was that it put Grant in position to become what he became-the top Federal commander, in which role he did what probably no other Union general could have done: grind down Lee's army in a nearly yearlong campaign of attrition, one during which much of the time he outnumbered Lee two or more to one and suffered almost twice as many casualties, until finally Lee, with his only alternative being the slaughter of his starving soldiers, surrendered and the war in effect ended.27
This, then, is what Grant's capture of Vicksburg accomplished in fact. But wait-there is one additional thing it did, namely to inspire the writing of a great many books, parts of books, and articles about Vicksburg with most of them, unfortunately, being in essence mere rehashes of its myths by authors obviously oblivious to its realities.28 And, quite likely, this will continue to be the case, for just as history tends to repeat itself, so too are writers of history inclined to duplicate what other history writers have written, for once an historical myth takes root it is vary difficult to pull it up and those who endeavor to do so should expect more in the way of resentment than compliment. Based on experience, I do."