Double Formula 1 world champion Sebastian Vettel will be stronger than ever this season after the most rigorous winter workout of his career, according to Red Bull owner Dietrich Mateschitz.Vettel dominated last season, winning 11 races with 15 pole positions and becoming the youngest ever double champion with four rounds to spare.Mateschitz said: "Sebastian has got better and is stronger than ever. He has trained over the winter break like he never did before now and he certainly won't disappoint."What he needs for the third title is a competitive car and we will be able to evaluate in the first tests in Jerez (in February) where the team is heading," he said.CONCORDE AGREEMENTMateschitz defended the decision to stick with Mark Webber, who won just once last year after being a championship contender in 2010, as Vettel's team mate.He said: "Mark is one of the best. His age is relative and he's one of the physically best prepared drivers...with a new number one mechanic at his side he will certainly not make life easy for Sebastian."Mateschitz expected McLaren, with Britons Jenson Button and Lewis Hamilton, to be Red Bull's closest rivals again, followed by Ferrari and Mercedes.Mateschitz added that he expected a new 'Concorde Agreement' between teams, commercial rights holder and governing body to be renewed soon and clarified the situation of Red Bull's sister team Toro Rosso."Toro Rosso has never been officially up for sale but we have always said that we would happily welcome important partners," he said.