That's an important bit of information which could easily explain why these buckles were found in Mississippi.

Since you mentioned the company's antebellum origins, it's possible that buckles were manufactured on order for the company. Being paid by private means, we can assume that not many in excess of actual need would have been ordered. Unless a roll from 1860 exists in a Vicksburg newspaper or the state archives, we don't know how many members the company included. However, state militia regulations state how many officers and men were required to form a volunteer company. For example, legislation passed for the Alabama Volunteer Corps (February 1860) called for a minimum of forty-four officers and men.

Do any of these buckles carry a manufacturer's stamp?

A name like "Vicksburg Sharp Shooters" tell us about arms carried by enlisted men. During the winter of 1859-60 Southern newspapers carried many articles about cases of newly-manufactured Sharps carbines seized at John Brown's farmhouse just over the Potomac River from Harpers Ferry. These had originally been purchased to advance Brown's anti-slavery operations in the Kansas Territory. Not to be outdone, governors of Southern states placed orders with the Sharps Rifle Manufactoring Co. of Hartford, Conn., the State of Georgia receiving 1,600 before the end of 1860.

Eventually the term 'sharpshooter' came to mean anyone regarded as a skilled rifleman. However, when the Vicksburg company formed it meant a man who carried a Sharps firearm. Arms purchased were likely breech-loading Sharps carbines, rifles manufactured by the Harford-based company being far less numerous.
http://americanhistory.si.edu/militaryhistory/collection/object.asp?ID=118

Serial numbers of individual Sharps arms manufactured -
http://www.american-firearms.com/american-firearms/z-html/company-S/Sharps%20Rifle%20Manufacturing%20Company/Sharps%20Rifle%20Manufacturing%20Company.html

An article about a "John Brown" Sharps carbine seized in October 1859. Many were appropriated on the spot by Virginia miltia companies. The rest were issued to the 1st U.S. Dragoons.
http://historical.ha.com/common/view_item.php?Sale_No=663&LotIdNo=16015