Victarion helps win the Shield Islands for his brother Euron, now the king of the Ironmen. The Shield Islands protect the inlet to the river that leads to Highgarden, so the Tyrells are going to be worried.
Euron is making everyone nervous with his madness: humiliating the defeated and selling them into slavery. But he has some political savvy, choosing as Lords of the captured islands those he wants to keep out of the way. He wants to go off in search of dragons and take over Westeros, and Victarion points out that it's a pretty difficult voyage for their longships. So he sends Victarion off on his own, in search of Dany, no less! What a match those two would make. She seemed to get on okay with the last crazed warlord she was married to, so maybe it is a match that could work. Euron certainly has more interest in Westeros than Drogo. And Dany could probably outsmart Euron enough to manipulate him. I think this plan just might work! Getting Victarion out of the way is pretty handy, too, since he wants Euron dead.
