Vandergrift, also known as The Visitor, was the commander of the Kaedweni forces in the last war between Kaedwen and Aedirn.
During that war, Henselt's troops, including the dreaded Dun Banner, crossed the Pontar the third day after the autumn equinox. Aedirn, led by Seltkirk, had good spies however so they were ready for the Kaedwenis whom they proceeded to decimate.
Vandergrift cut Seltkirk off from the rest of his army. The two met in the middle of the melee. It was a battle of titans. In the end, the Visitor hacked Seltkirk and sliced the white knight from his head to his balls. A terrible death that sent the Aedirnian ranks into disarray.
During the battle, Vandergrift's long simmering disdain of Sabrina Glevissig came to a head and the two argued bitterly which led the sorceress to retaliate by causing balls of fire to rain down on Kaedweni and Aedirnian combatants alike.
Earlier, in peace time, Seltkirk and Vandergrift had met at a tournament in Ard Carraigh. Seltkirk won then. Vandergrift's sword cracked and the Visitor had to yield.
In The Witcher 2: Assassins of Kings, Vandergrift only appears as the draug.