More information from Bearss, Vol 2.
Page 567, May 14, "Pemberton, having resolved to move on Dillon's, called for the trustworthy Captain Yerger. At 5:40 p.m. he handed the captain a dispatch addressed to General Johnsoton, informing his superior of the impending march on Dillon's, and of his plan to cut Grant's communications." [Message found in OR, Pt. 1, Vol. 24, Pt. 3, p.567]
Page 568, "On May 14, while Pemberton procrastinated and his junior officers argued, Confederate arms had suffered a severe setback at Jackson. Late that afternoon, McPherson had contacted Grant and shared with him the copy of Johnston's May 13 dispatch to Pemberton, which had been betrayed into Federal hands. The Union chief now had what he thought was the key to the Confederate plan of operations. Source: Stephen D. Lee, "The Campaign of Vicksburg, PHMS, III, 33-4" I do not know of the date of Lee's work, perhaps he was quoting Grant's1884 Memoirs.