Well, the traditional viewpoint is that Vicksburg pretty much broke the Confederacy in half by finally ceding the Mississippi to Union control. There could be no transportation of resources from West to East and no use of the Mississippi River anymore. A revisionist argument is that the Union already had control of much of the Mississippi anyway because they had Port New Orleans and that Vicksburg's value was more in raising Northern morale and making it look the Union was doing something than anything tangible. It was also the success of Vicksburg that put that guy whose name I can't quite remember but who comes up here every now and then in command of the overall Union army. It was a two-punch with Gettysburg- both showed the North that they could win victories after the devastating defeats they had suffered and Gettysburg would insure that Lee would no longer try to invade Northern soil.
I don't know if they were turning points per-say, but they definitely both seemed to have galvanized the North to thinking that they could do this.