this is directly from the wowpedia article on vandel: "Vandel is a high ranking demon hunter of the Illidari. He was a night elf who lived in Ashenvale, but during the Third War his village was destroyed by the Burning Legion, and his son, Khariel, was torn apart by a felhound. Vandel then killed the felhound with a knife, and encountered Illidan Stormrage when he stumbled out of his house. Believing the Betrayer to be behind the attack, Vandel attacked him, but Illidan easily fended off his attacks and asked the night elf to serve him, impressed that he killed a demon with only a hunting knife. When Vandel refused, Illidan told him to seek him out eventually, if he truly sought vengeance.
Eventually, Vandel did seek out Illidan, finding a way to reach Outland and scaling the walls of the Black Temple. Illidan readily welcomed him into his service, and told him he would find what he sought or die trying."
so after illidan went to outland but before We The Adventurers came along, since the same article details the dark portal being re-opened and the alliance and horde forces marching through later on.
Re: op
Eventually, Vandel did seek out Illidan, finding a way to reach Outland and scaling the walls of the Black Temple. Illidan readily welcomed him into his service, and told him he would find what he sought or die trying."
so after illidan went to outland but before We The Adventurers came along, since the same article details the dark portal being re-opened and the alliance and horde forces marching through later on.