At the start of the American Civil War, Vincent joined the Pennsylvania Militia as an adjutant and first lieutenant of the Erie Regiment. On September 14, 1861, he was commissioned lieutenant colonel of the 83rd Pennsylvania Infantry and was promoted to colonel the following June. After the death of his regimental commander in the Seven Days Battles (at the Battle of Gaines's Mill), Vincent assumed command of the regiment. He developed malaria on the Virginia Peninsula and was on medical leave until the Battle of Fredericksburg in December 1862. On May 20, 1863, he assumed command of the 3rd Brigade, 1st Division, V Corps, Army of the Potomac, replacing his brigade commander, who resigned after the Battle of Chancellorsville.