Vabbi fell because they were caught by the overwhelming forces of Joko, who then diverted the Elon river making both Kourna and Istan submit to him.
Kourna I understand, but what about Istan? It's an island with quite possibly the largest fleet in the region, home of the Sunspear oder, and they don't depend on the Elon for water. It has the military, supplies, and geography needed to survive.
Why did Istan yield? Any guesses?