The quote is from Wikipedia but I've seen the battle described that way before. Was Vicksburg the last major crossing point of the Mississippi that the Confederacy controlled? Also I would think that if there were enough soldiers west of the Mississippi to make this a serious blow, then there would be enough soldiers to try and retake one of the other crossing points. I'm just looking for a little context on this battle.
You are correct. Vicksburg was the last crossing point of the Mississippi controlled by the Confederacy. (There were at this time no bridges. Crossing was by steamboat and or barges. That is, there were no bridges crossing in Confederate territory. There was a railroad bridge between Illinois and Iowa over the Mississippi at Rock Island, which was completed in 1856.)
US gunboats moving upstream from New Orleans controlled the river south of Vicksburg. US gunboats moving downstream controlled the river north of Vicksburg.
The big Confederate guns at Vicksburg kept the US gunboats away from that section of the river. Under the protection of those guns, Confederate troops and supplies could cross the river in volume. (Small boats could probably cross the river at many points, as the US gunboats could not be everywhere, but they could not transport much.)
Once Vicksburg fell, the Confederacy was split in two. Neither the West nor the East could reinforce one another. Supplies (cannon and munitions from the East, beef and cattle from the West, for example) could not be sent in either direction.
No amount of Confederate soldiers on either side of the river could have recaptured a crossing point. The river was dominated by US warships. Only a secure fortress, mounting many powerful cannon, could create a safe zone for Confederate crossing of the river. After Vicksburg, Confederate ability to create such a fortress, and mount powerful cannon along the Mississippi was nil.
Most of the soldiers that were west of the Mississippi before the fall of Vicksburg crossed into Mississippi with General Earl Van Dorn after the Confederate/Missouri State Guard loss at the Battle of Pea Ridge in NW Arkansas. On their way there, General Van Dorn stripped Arkansas of all military supplies, including manufacturing equipment. After they arrived, the Arkansas State Government petitioned the Confederate Government for a new army, since there was basically less than 5,000 regular troops in the entire state. They sent General Thomas Hindman. He was able to raise another army (Around 10,000 strong) but was poorly supplied. To make a long story short, he called on all "loyal" citizen to start harassing the Union troops in NW, NC and NE Arkansas. This started a huge guerrilla war in the state. After his loss at the Battle of Prairie Grove, the army all but dissolved. The rest of the forces in Arkansas were too busy fighting the Union army in the North of the state to help Vicksburg much. They tried relieve some pressure by attacking a small but well defended city called Arkansas Post that is near the Mississippi River, but it was too little, too late. Vicksburg had already fallen. The Confederate army in the west was too small and too ill-equipped to help. The beginning of the end was already here.