way I figured it, is in the beginning of killzone 3, Vekta is withdrawing and ordering their troops to surrender since the Helghast main force arrives and starts wiping the floor with them (and Rico killed Visari, whom he had orders to capture, not kill, since assassinating the only man the Helghast listen to is probably a bad idea, and then trade Visari for Helghan's surrender and/or force him to sign a treaty) Sev and a band of misfits decide surrendering to the people they just invaded and murdered their Autarch is probably a bad idea. So basically all the stuff that happened in 3 is them acting 'Rogue' since they weren't even supposed to be there. Lo and behold, Jorhan Stahl is planning to blow up Earth, and when Sev and pals stop him, it blows up Helghan.
Now earth steps in, oblivious to Stahl's plan (not to mention Stahl was acting without the consent of his government, anyway) and goes "holy fuck you just genocided their planet? AFTER assassinating their leader in cold blood? and you were acting against orders the whole fucking time? nah, hell nah, you're going to have to pay for that." so they give half of Vekta to Helghan for reparations (I guess, best I can figure).
As for not leaving as the wall was going up...no fucking clue. Either A) the native Vektans are retarded and didnt leave in time, or B) the Helghast decided they needed a workforce and started stopping people from leaving as the wall was going up, like the soviets did.