Part of the plan was for Grant to occupy and hold Pemberton in the north while Sherman advanced down the river route. Sherman states below that this (communication and cooperation with Grant) was essential to the overall success of reaching Vicksburg in this early move. He also states that the plans changed before his assault, but he went ahead with it only because these changes had not reached him.
Both Sherman and Grant indicate that no assault would have been made on Chickasaw Bluffs had the two been in communication. Both also indicate that the Van Dorn Raid on Holly Springs had a profound effect on the early stages of the Vicksburg Campaign, and was the singular event that made Grant retreat from Mississippi to Memphis.
From Sherman’s Memoirs:
"It afterward transpired that Van Dorn had captured Holly Springs on the 20th of December, and that General Grant fell back very soon after. General Pemberton, who had telegraphic and railroad communication with Vicksburg, was therefore at perfect liberty to reenforce the place with a garrison equal, if not superior, to my command. The rebels held high, commanding ground, and could see every movement of our men and boats, so that the only possible hope of success consisted in celerity and surprise, and in General Grant's holding all of Pemberton's army hard pressed meantime. General Grant was perfectly aware of this, and had sent me word of the change, but it did not reach me in time; indeed, I was not aware of it until after my assault of December 29th, and until the news was brought me by General McClernand as related."
From Grant’s Memoirs:
(To Sherman, December 8, from Oxford. ) “Inform me at the earliest practicable day of the time when you will embark, and such plans as may then be matured. I will hold the forces here in readiness to co-operate with you in such manner as the movements of the enemy may make necessary.
“It was understood, however, between General Sherman and myself that our movements were to be co-operative; if Pemberton could not be held away from Vicksburg I was to follow him; but at that time it was not expected to abandon the railroad north of the Yallabusha. With that point as a secondary base of supplies, the possibility of moving down the Yazoo until communications could be opened with the Mississippi was contemplated.
“It was my intention, and so understood by Sherman and his command, that if the enemy should fall back I would follow him even to the gates of Vicksburg. I intended in such an event to hold the road to Grenada on the Yallabusha and cut loose from there, expecting to establish a new base of supplies on the Yazoo, or at Vicksburg itself, with Grenada to fall back upon in case of failure.
“On the 20th General Van Dorn appeared at Holly Springs, my secondary base of supplies, captured the garrison of 1,500 men commanded by Colonel Murphy, of the 8th Wisconsin regiment, and destroyed all our munitions of war, food and forage. The capture was a disgraceful one to the officer commanding but not to the troops under him. At the same time Forrest got on our line of railroad between Jackson, Tennessee, and Columbus, Kentucky, doing much damage to it. This cut me off from all communication with the north for more than a week, and it was more than two weeks before rations or forage could be issued from stores obtained in the regular way. This demonstrated the impossibility of maintaining so long a line of road over which to draw supplies for an army moving in an enemy’s country. I determined, therefore, to abandon my campaign into the interior with Columbus as a base, and returned to La Grange and Grand Junction destroying the road to my front and repairing the road to Memphis, making the Mississippi river the line over which to draw supplies. Pemberton was falling back at the same time.
“This interruption in my communications north—I was really cut off from communication with a great part of my own command during this time—resulted in Sherman’s moving from Memphis before McClernand could arrive, for my dispatch of the 18th did not reach McClernand. Pemberton got back to Vicksburg before Sherman got there. The rebel positions were on a bluff on the Yazoo River, some miles above its mouth. The waters were high so that the bottoms were generally overflowed, leaving only narrow causeways of dry land between points of debarkation and the high bluffs. These were fortified and defended at all points. The rebel position was impregnable against any force that could be brought against its front. Sherman could not use one-fourth of his force. His efforts to capture the city, or the high ground north of it, were necessarily unavailing.
“Sherman’s attack was very unfortunate, but I had no opportunity of communicating with him after the destruction of the road and telegraph to my rear on the 20th. He did not know but what I was in the rear of the enemy and depending on him to open a new base of supplies for the troops with me. I had, before he started from Memphis, directed him to take with him a few small steamers suitable for the navigation of the Yazoo, not knowing but that I might want them to supply me after cutting loose from my base at Grenada.”
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"Pemberton was still being successfully held on the Grenada line."
Not according to Sherman.
"Sherman would have had no reason to call off the assault."
The reasons are stated above by Grant and Sherman.
"Grant had standing orders to go no farther south overland, and had stated that he did not have sufficient force to move past Grenada."
Grant stated that if Pemberton began to fall back he would follow him to the gates of Vicksburg. The Union forces were far superior to Pemberton's.
"Grant would have returned to Memphis and assumed command of the riverine campaign, Van Dorn or no Van Dorn."
That was not the plan. There is nothing anywhere to suggest that Grant would enter Mississippi to push to Grenada in order to stop, turn around, and go back. Grant credits the Van Dorn raid as the impetus for “abandoning my campaign into the interior.”
"Grant was still entertaining the option of moving to meet Sherman, but that option was wholly contigent upon a victory by Sherman."
And both generals state that such a victory was contingent upon Grant and Sherman staying in communication and cooperating in the advance.
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I'm not trying to propagate an argument, but I remembered the two memoirs and went back to look up some of the particulars for verification, which I pasted here. All of this can be found on line doing a search for Grant's and Sherman's memoirs. I selected the pertinent information, so what I pasted is not always line by line as in the original writings. It is, however, direct quotes presented in context.
I'd never heard that Van Dorn's raid was a minor incidental to Grant's presence in north Mississippi, and that most everything else would have been the same with it or without it. Grant and Sherman mention it as having quite a bit of importance.