I can't see it well from the photo, but it looks like an eagle button. If there is a V in the shield, it's a Voltigeurs button. The Regiment of Voltigeurs and Foot Riflemen was a one-year regiment of the United States Army raised during the Mexican-American War. It was active in 1847 and 1848.
It is believed that surplus Voltigeurs buttons were used by Vicksburg militia units and maybe some Virginia militia units and carried into the civil war.