Vulkan would not join his legion until several years after their discovery. When he did, it would be when they needed him most.
Via our good friends at Lexicanum
When Vulkan came to his Legion, it was in the hour of their need. The XVIII, led by their Lord Commander Cassian Vaughn had become embroiled in the defence of a cluster of worlds near the Taras Division against a horde of Orks. The XVIII was the only Space Marine Legion able to respond to the crisis. Fighting against vast and overwhelming odds, 19,000 Space Marines held out against millions of Ork raiders and their fleet of Roks. The actions of the Legion had allowed the evacuations of three entire planetary populations to the nominal safety of the Taras System, but at a terrible cost. Their Primarch, however, learning of their plight refused to stand by and instead joined the fray.
When Vulkan arrived he did not do so alone, he brought with him 3,000 new initiates and equipment from Nocturne. The reinforcements brutally fell upon the Orks, spurring the surviving Space Marines to counterattack. Caught between this hammer and anvil of savagery that over-matched their own, the Ork horde was broken and put to flight, and the survivors were relentlessly pursued and consumed by fire. In the aftermath, the two halves of the XVIII Legion met and were unified when Vulkan finally revealed himself to his sons. The survivors of the XVIIIth Legion knelt immediately, but Vulkan bid them rise, saying that all his sons were equals and he was no petty king needing shows of obedience. Instead, it was he who knelt in honour of the lives they had saved and the price they had paid. Then, seeking out the mortally wounded Lord Commander Vaughn, he conferred the formal transfer of the Legion’s mastery by presenting the fallen warrior with the broken power klaw of the slain Ork Warlord.
The Salamanders were apparently reorganized by Vulkan upon his discovery and assumption of command, with that organisation said to still be in effect in their current form, with seven companies to a Chapter, each company being founded by the seven different great settlements of Nocturne and commanded by a Captain from that settlement. The number of Chapters in the Great Crusade era Legion is currently unknown, but they had at least 34 Companies. The Salamanders are often said to have “always been” the smallest legion but how this claim stacks against the paucity of both Emperor’s Children and Thousand Sons during their respective histories is not yet known. Notable campaigns of the Salamanders waged during the Great Crusade were the Compliance of Kharaatan and Conquest of One-Five-Four Four.As far as can be determined by the Magos Biologis of the Adeptus Mechanicus, the Salamander’s gene-seed appears to be both stable and as yet uncorrupted. An unusual trait of the Salamanders is that their Battle-Brothers tend to be slower in reflex reaction than other Chapters, though the origin of this factor is debated; it is unknown whether this defect is due to a problem with the gene-seed that manifested as a result of most Salamanders being raised on their high-gravity world, or the psychological result of the Chapter’s doctrines and psycho-conditioning against hastiness and impetuosity. However, it has been noted that a Salamanders Space Marine can move just as quickly as any Astartes equipped with Power Armour, and are still significantly faster than those of a normal human. Also, as a result of a reaction between their genetics and the high levels of radiation on Nocturne, Salamanders battle brothers have dark or jet black skin and bright, burning eyes. This frightening appearance is entirely superficial, but has intimidated more than one rebellion into submission without firing a shot. Astra Militarum Combos You Need To Know Episode 122