Don't forget that its more than just the NPS site. Was there last summer and they have done great work at removing the Kudzu and clearing a good third of the battlefield to the historic vegetation state which makes it much more interesting. Head over to Champion Hill (run by the state and hopefully eventually annexed into the Vicksburg NPS Unit.) as well because I think that is where Grant really won the battle. Missouri Monument is very nice and there are at least two locations where Missourians fought each other, truely brother against brother, I think they have primary documentation of 3 sets of brothers that were directly engaged against each other. The artillary demo is well done and worth waiting for if you have someone with you that has never heard the roar of battle.
John R.