Keep at it. I finally found Yag in my world. Found the Vegistir at a henge with 2 Beserkers. Are you sure you didn't miss it? It was the 4th plains I searched. 2 which were really huge. Now its time to lay a beatdown on it. Funny thing once I sailed to the altar island I found, within seeing distance of the altar, another Veg on same island. Seems like a waste to me.