We agree on some things, but not on others.

Just for sake of discussion, I think I need to mention that my source is DGP 'Vilani & Vargr '... I got rid of a bunch of wargaming and rpg stuff years ago, so can't look things up in them anymore.
***I don't see why they would necessarily unite in the face of alien invaders. They might, but then again, they might not. But it's moot, because the Imperium isn't invading them during the Rebellion. For the Vargr, it must be self-interest (enlightened or not as the case might be) all the way. ***
Agreed, the Imperium didn't invade them, however, the idea that an incursion into Vargr space could cause them to act united against a single enemy instead of fighting amongst themselves would bring about fears of territoriality concerns.
***[*] BTW, nor really a fair description of what is clearly stated to be the
biggest raiding band around.***
From my source, the Kforuzeng were most powerful corsair band in the Gvurrdon sector. They absorbed several other bands, among them the Aegzaeng, who provided ground forces and hired out as mercenaries. The Kforuzeng began to splinter prior to the rebellion with the Ethueng breaking away and hired by Tukera as escorts and raiders against Tukera competitors. If not for the Oekhsos tirades and other anti-Imperium propaganda, they would have disbanded permanently by 1115. That description hardly fits the pitiful order of battle mentioned in earlier posts. At least not in the big-ship OTU.
***Yes, but that's where I feel the implausibility lies. The Vargr invaders would logically be a limited part of a limited slice of the Vargr Extents (And the further away a Vargr world is, the more limited the percentage of ships they will 'supply').***
***The Domain, OTOH, can call upon the massed forces of 34 subsectors. Granted, in 1117 Norris orders most of those forces to assemble in Vilis to stave off the Zhodani assault he expects, but, not being an idiot like Lucan, is he really going to strip the subsectors that borders on a rapacious raider culture completely bare? These worlds supply him with the resources to field the forces he needs to stave off the Zhodani. If the Vargr takes over a world, he loses the taxes he needs to keep his ships flying. Moreover, the worlds will have fixed defenses that won't be removed.***
In Corridor, the Dzarrgh Federate was a close trading partner of the Imperium, but when the Rebillion occured, they endorsed looting Imperium territory and even sent their own military assets to do so. The other main group to do so were the 'Glory of Taarskoernz' which failed to take Depot. A corair group, 'the Vaenggvae' acting as mercenaries to them succeeded in coercing Depot's personnel and thus becoming so powerful that it could ( and did ) dictate policy to its former employers. This group was present, with humans, at important negotiations concerning territorial disputes which ended with the Dzarrgh Federate controlling most of the worlds in the Corridor. It is suggeted that Denebian interests were involved with Vaenggvae interests.
Given that Norris pursues an isolationist position during the Rebellion, the closing of Corridor in beneficial in that it prevents the chaos of the Core from upsetting his domain. Norris is, indeed, not an idiot like Lucan.
***I submit that whatever glory days the Vargr may have had will be over at this point.***
And I submit that the glory days of the Vargr are unchanged.
On an idea closer to the topic here, what about Impie pirates crossing into Vargr space for raiding? If there is profit to be had with little risk, someone be doing it. So far, the consensus is that the Vargr are not a real threat, so Impies should have an easy time. After all, Impie pirates in Imperial space will have batrons chasing them down, eh?
Hans is quoted by statements marked with asterisks
I was too dumb to figure out 'quoting'..sorry for any confusion