Been playing the newest GameOfThrones Mod, but I have questions using "Pay The Iron Price"...

So, I recently downloaded the newest version for the GameOfThrones Mod, and I started out playing as Balon Greyjoy in the War of the Usurper scenario. I chose the Greyjoys because I'm not that good, they're independent and they're rather isolated. I used the "Pay the Iron Price" CB to declare war on King Aerys about 8 years ago because he was busy fighting off the North/Riverlands/Vale/Stormlands. I went around and took over some counties, but when Rhaegar took over the Throne, he sent me a peace offer that I felt was good (~250 gold). So I accepted, but I noticed that I didn't keep any counties. Whatever, I thought, the lands were literally on the far side of the map, so no big deal.

Around 8 years later, Seagard and the Riverlands are looking tasty! Hoster's imprisoned (Rhaegar is definitely going to win this war), the Riverlands' army is shattered, and I want to expand my territory. So I can only use the "Pay the Iron Price" CB to declare war (no marriage claims), but I'm worried that I will lose counties again if I accept a peace offer. Do I have to completely conquer the Riverlands to keep the lands? Or can I just conquer a few places until the warscore allows me to "enforce demands"? Actually, do I even have demands... paying the iron price is basically pillaging for fun...