The message, in a vial with a bullet, was written in the Vigenere cipher" in which letters of the alphabet are shifted a number of places so that an "a" in the uncoded text becomes a "d" in the coded document. The date on the coded document is July 4th.
You can expect no help from this side of the river. Let Johnston know, if possible, when you can attack the same point on the enemy's lines. Inform me also and I will endeavor to make a diversion. I have sent some caps. I subjoin a dispatch from General Johnston."
Pemberton surrendered the city to Union forces on the same day that the cipher was sent.
Vicksburg was so scarred from the experience that the residents refused to celebrate the Fourth of July for the next eighty years.
Please see Battle of Vicksburg to learn more.
[Source: "Civil War Letter Decoded: No Help on the Way," Steve Szkotak, Associated Press]