First of all, I don't believe having very little dragon blood[bBP and Qyentyn], or even blowing the dragon horn, will have any control over Rhaegal and Viserion, and the fact that I don't believe Vic has the real horn, doesn't even come into question. I just simply don't believe that the horn can control a dragon. In fact, I believe that it will enrage them even more. To be honest, I am starting to believe that only Dany will be able to control the dragons, because she is their "mother". So far, we have Quentyn, who also has some drops of Targ blood in him, and yet he was fried[i don't think he's still alive]. Now, in the past, whenever Tyrion is feeling pretty miserable, something happens almost instantly that changes his fate, from GoT(Bronn deciding to champion him when he was a goner in the Vale), Oberyn doing the same against the Mountain later, Jaime coming to free him when he was to be executed. Who's to say that Tyrion miserableness isn't a hint from GRRM that Tyrion's fate will change sooner rather than later. I don't believe Dany needs a horn to control the dragons that she herself hatched from eggs, and Vic's point in going to Meereen is to aid Dany against the slavers, probably bring Moqorro to her, and bring ships as well. I believe that in fear of someone else controlling the dragons, he will blow the horn himself and die for it. But I also believe that Ser Barristan and Tyrion are in the need of a chat. That's why I believe that Barristan will survive this battle. Dany, for instance, still has a lot to learn from him that she will not learn from anyone else, especially not from Tyrion, because I don't think Tyrion will ever have anyone else's but his own best interests at heart(he used to have Jaime's too, but no more).