G. Kashkanun Anderson
SOC-13
The Vargr recruiting occurred in Corridor and Provence Sectors, right around the time that the Ziru Sirka was just past its prime, but not obviously in decline yet. Think early Third Century Rome. It's also worth noting that Vilani-Vargr interaction was happening in exclusively Makhidkarun Bureau space throughout the entire First Imperium period. Makhidkarun evolved from Vland's old noble castes, and I imagine that put a certain tenor to their leadership culture. The local human population might have come to the conclusion that certain jobs were 'beneath' them, particularly in a society who's sense of obligation is fraying on the edges, and if another group of outsiders is seen as filling in those gaps.The expansion into the Vilani Imperium, with governors equipping and using Vargr mercenary bands may be an exception, rather than a rule, because with interstellar populations, you'd surely find enough candidates to fill your security forces ranks.
The Vargr had about a thousand years to chip away at the Ziru Sirka in Provence, Corridor and Deneb Sectors. With a time frame like that, you can overrun the Vilani with simple demographics. And the Vargr also had the technological advantage after -2725, when they acquired Jump-3 (and God knows what else) from the Zhodani.The Vilani must have been exceptionally weak, and lack the capability of self defence if the Vargr managed to overrun them, after which the Vargr should have been attritioned through any number of revolts and insurgencies.
As for the rest of the Extents (Meshan, Mendan, Gashikan and the like), the Vargr didn't cross over the Coreward side of the Windhorn Rift until after about -2400. Imperial authority was seriously curtailed in the Coreward provinces by then, and there really wasn't any centralized authority to oppose Vargr expansion in the region until the founding of the First Empire of Gashikan in -1784. Once Gashikan got its act together via the genocidal Second Empire, the Vargr in the region were at the pointy end of a full-blown extermination campaign until the rise of the Julian Protectorate in the 100s.