I am a Texas Historian, and I think I can offer some information on this. Vicksburg was indeed a major defeat for the Confederacy, but the Union did not hold the entire Mississippi afterwards.
The Confederacy still held the lower portions of the Mississippi, but they could no longer use the upper portions of the river for transportation or communications. So, if you can envision the Southern states, there was still a narrow opening below Vicksburg and the gulf.
In Texas, after Vicksburg, communication with Richmond was hard to come by. The state government was basically defending itself, and was relying on the combined Confederate forces of Arkansas, Louisiana, and Texas.
In late 1863, the Union launched a major two pronged invasion of Texas commanded by General Banks. The intention of these campaigns was to:
Completely sever the Confederate overland cotton trade into Mexico.
Seize and capture the Texas coastline, including its coastal defense systems, and reduce the amount of blockade runners.
2,000 Union soldiers came ashore at the mouth of the Rio Grande and separated into two columns. The first movement of troops started up the river and eventually made it all the way up to Laredo before a Confederate garrison, under the command of an Hispanic officer, halted their advance in an astonishing victory.
The second, and more successful column of Union soldiers, began moving up the Texas coastline towards Galveston. These troops were able to seize all of the Barrier Islands, the majority of the port cities (Corpus Christi, Indianola, Port Lavaca, etc.), and all of the Confederate fortifications from the mouth of the Rio Grande to the tip of Matagorda Island.
So what does all this rambling have to do with your original post? The Union invasion of Texas, which lasted from 1863-1864, generated a massive scare with-in the western portions of the Confederacy.
When the Federals began their campaigns in Texas, the Rebel leaders of the western Confederacy panicked. Separated from the majority of the South, with an extremely low number of soldiers (most of whom were already losing hope for a Southern victory), the commanding officers had to go as far as calling up all of the local militias and volunteer units to aid in the defense of the state.
By the Spring of 1864, there were almost a 1,000 Union soldiers ready to make an attack against either Galveston or Houston. But, for some untold reason, Banks suddenly decided to cancel the movement and ordered a campaign to seize control of the Red River in Louisiana. It would prove to be a major mistake.
The Red River Campaign of 1864, was aimed at furthering the rift with-in the Confederacy. The Union had finally seized control of New Orleans, and Banks foolishly believed that northern Louisiana and eastern Texas were mostly devoid of Confederate forces. His error in judgement led to the Federal defeat at the Battle of Sabine Crossroads, which altered the entirety of the campaign, and re-fueled the Confederate morale.
If you want to find primary documentation on just how scattered and panicked the Confederate forces in the west were after Vicksburg, I highly suggest researching the Texas campaigns of 1863-64. Probably one of the best books on the subject is titled “The Yankee Invasion of Texas”. I can’t recall the name of the author, but I know you can find it on Amazon for about 20 bucks.
Hope this helps!
Sorry, guess I was typing too fast. I guess I should have worded it as follows: “From New Orleans, Banks believed he could advance into Louisiana virtually unopposed by the Confederate forces in eastern Texas and northern Louisiana.”
It happens. But, for those interested, here’s a good article on the Red River Campaign: