Lee's invasions of the North didn't have a good track record. Sharpsburg was a shambles when he unnecessarily split his forces before the enemy then had to quickly regroup with their backs to the Potomac. But the ANV was at it's best by mid-63 and their spirits high after Fredericksburg and Chancellorsville, so there is a good chance that Bobby Lee might've learned the lessons past and made the victory Confederate independance demanded: the capture of Washinton and free reign in the North-East. Vicksburg wasn't nearly as important as the North thought it was. The city was written off by Richmond months ago, the Confederacy was already cut in half, with or without its capture. If anything it was the loss of 60 000 CS soldiers that hurt the most. So we have a pretty dire picture for the US: Grant regrouping in the Mississippi Valley, Rosecrans stuck at Chattanoonga and Lee moving as he pleases between Philadelphia and Washington. Britain steps in; wars over.
But if Lee LOST an invasion of the North I can only imagine that would speed the Confederacy's demise. This was the only real chance to strike before the US fully flexed it's muscles and the CS victories of '62 and early '63 still in the minds of all. Either way it would have been a shorter war instead of the meatgrinder it became in OTL.