Sorry but I for one can't get along with the "revenge" issue. Vrll won't for sure want or apply for wolrd peace on Earth, and certainly won't care for which side they're fighting against, but just fight whoever happens to be in charge of the area where the resources/devices they want to retrieve are. Also, there are two things to think about. The Vrill else have both weapons terrible enough as to wipe out entire areas of territory and no qualms about using them or they have some kind of limit to the sheer destruction they can cause, be it because of their weapons not being destructive enough, the capability of their troopers high enough or their interests or ethicals overweighing the possible caused destruction.

The first case is obviously out of question. The last case being the chosen, most surely Vrill would send in their warriors to protect their VK sources, possibly in small forces just enough for that purpose. Just as we have seen for the last 8 years in Iraq, where besides a geo estrategical option to change a liability with a mostly favourable government there was an economical interest and a will to protect it. (Note I am not speculating but just acknowledging something that happened). While we must understand that Vrill mindset will be differing from human one, shared experience demonstrates both that it is not much so and that it cannot be spared some interests that humans have. Such one is the acquisition and control of resources for own sake.

In 1947 there is no one common enemy for SSu and Allies but three sides fighting each other. The (even if sudden) manifestation of a fourth contender could be welcome as a possible enemy of my enemy. While some kind of agreement could be had with one contender to have a share of Vrill resources, it would surely become so only in a subject condition. One thing I am sure of: every one of the three factions will look upon Vrill as aggressors, just as they look upon the other two...

 Hmmm i just had a terrible thought . What if the Vrill won't be an army? What if they just come and let's say ally themselves with the SSu providing armor 4 troops and hvy walkers?:D Or that there will come sub fractions of Vrill that will join the cause among each of old fractions:D I know, sick...:P

That would actually be VERY cool. I have no need for new factions. But a new group that can boost the abilities of the factions I play with, that sounds very interesting!

The Vrill as raiders working with all factions is probably the best way to get them fighting everybody, without the factions uniting against them.

They could insist on independent control, which would mean Vrill only forces, but it could be interesting to see them fielded in normal factions.

The down side to that would be it would mean Vrill fighting Vrill, which wouldn't make sense with a limited current supply of Vrill.  Perhaps set up a bidding system where only one side can field Vrill in a battle, but that would be rough for tournament play.

Independently controlled Vrill forces facing other Vrill could have a 'skirmish' that drove off the opposing Vrill force and left the Vrill to collect the objective.  That no Vrill were lost to the winners, or false casualties quickly replaced by other Vrill transfered from other units, would leave few questions.  Any allied observers at the action would have to be amongst the casualties, but they wouldn't have Vrill armor to protect them.

The Vrill might have weapons that could cause massive destruction, but those weapons could easily have limits.  If they feared detection by their current opponents, they could be keeping their ammunitions stores for their big guns to use against them.  Just how big those big guns are is also open to question.

We know the Vrill have interstellar travel capabilties, and can assume they at least have nuclear power, but we have no clues what else they have.  We don't know if they translate their ships into a warp space for FTL travel, or have found a way to do so in our universe.  They may be restricted to jump points for interstellar travel, and so have only specific routes they can travel.  They may use weapons of mass destruction, or they may limit their ships to energy weapons that are more reliable and don't require ammunition stores.

We know less of Vrill culture.  The may see revenge for killing one of them as appropriate, or they may praise the person who did it.  We know they can use subtrefuge, as they were willing to lie overtly and through omission, but we don't know if that was because of the situation, or an accepted racial trait for every day life.

Introducing the Vrill into the game will not be a problem.  Everyone already knows that their numbers will be limited because of the problems they have been experiencing in wormhole technology.  That same set of problems is also what is limiting them from bringing their biggest and heaviest warships into the battle.  Otherwise, with their technology they could easily destroy human civilization.  If they can gather up enough of the VK resource, they might be able to reduce the amount of destabilization their wormholes are experiencing on this end and that would allow more Vrill and their “bigger” weapons of war into our realm. That would be bad for all human factions.  However, the threat is not eminent enough for humanity to forgive and forget their grudges against each other.  So, there will be no truce or possibility of a “human” coalition against the Vrill until much later if ever in the story.