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Short TL;DR taken from Autosport forum
Vettel to RBR is a real possibility ( he had long private talks at Hungary with Red Bull bosses), he would also get out of his Ferrari contract as he according to TS has a get out clause (not in top 3 in drivers points).
Apparently Vettel is unhappy with Ferrari’s development path (sacrifice downforce to get more straightline speed) and he thinks he would be achieve more with RBR.