After his return to Sweden, he was awarded the brevet grade of Brigadier general of volunteers and in 1893 he received the Medal of Honor for bravery in the Battle of Gaines's Mill.

At the outbreak of the Civil War in 1861, von Vegesack left for the United States and joined the Union Army.

As an ADC to Butterfield he took part in the battles of Hanover Court House, of Seven Pines and of Fair Oaks.

In May 1863 Vegesack's regiment was a part of Joseph Hooker's left wing in the Battle of Chancellorsville.

[5] On February 22, 1866, President Andrew Johnson nominated von Vegesack for award of the brevet grade of brigadier general of volunteers, to rank from March 13, 1865, and the United States Senate confirmed the appointment on April 10, 1866.

After his return to Sweden, Vegesack was promoted to lieutenant colonel and given the command over Västerbotten Field Jäger Corps in 1864.

[1] From 1878 till 1887 Vegesack sat in the upper house of the parliament of Sweden as a delegate for Gotland.

Citation: While voluntarily serving as aide-de-camp, successfully and advantageously charged the position of troops under fire.