Although Renault now, with Ricciardo & Norris confirmed at McLaren, seems to be the only halfway competitive seat available, IMO Vettel would be crazy to take it. Renault haven't been getting their shit together for quite a while now, neither in the engine nor the chassis department. Their performance last year was absolutely dismal, in spite of two very talented drivers. In addition, Seb would have to face French favourite Ocon as a team-mate. This would be a repeat of the situation at Ferrari last year, maybe worse, as Ocon does have a history of feuding with his team-mate.
About the only thing that speaks for Renault is that Vettel scored all of his championships powered by engines from Viry-Châtillon