LONDON, England - Sebastian Vettel's contract extension with Red Bull must have caused some worry among rivals - it sends a clear signal that the triple World champion and his Red Bull team are looking for even more success.Vettel earlier has signed a contract extension that will see him with the team until the end of 2015 and should allow him to fully concentrate on winning a fourth consecutive championship.The contract extension ensures stability for Red Bull as the important members of the team, such as team boss Christian Horner and designer Adrian Newey, also have long-term contracts.HAPPY CHAPVettel said: "You have to be happy in your team and have a competitive car. At the moment I have both." His signing the contract should also put an end to ongoing speculation that the German championship leader will soon be moving to Ferrari or Mercedes.Instead, 25-year-old Vettel continues to trust the team which supported him at the beginning of his career.He is the clear No.1 and has an established team working with him as he helped Red Bull become only the fourth team in F1 history to win at least three consecutive Constructors' championships.At the beginning of the 2013 season Horner extended his contract until 2017; Newey, widely credited with Red Bull's dominance, has resisted all attempts to lure him away."He gives me the confidence and designs the best cars in the world," Vettel has said. "It is a great experience to work with him and I think we could become even better."GOOD THING GOINGNewey was one reasons why Vettel extended his contract, believing that the Briton would also be able to continue to build very competitive cars after wide-ranging changes to the regulations are introduced for 2014, mainly that the engines will be V6 turbo units that will result in changes to the energy recovery system and fuel restrictions.Vettel could have left the team at the end of 2014 if he though 2014's car would not be competitive.Red Bull would presumably like to keep Vettel under contract until he retires. "He is becoming stronger and better," Horner said after Vettel won in Canada, adding: "The impressive thing is that he continues to strive to better himself. He wants to achieve as much as possible."One question remains: Vettel's team mate for 2014. Mark Webber's contract ends will run out at the end of the 2013 season and it seems likely that the 37-year-old Australian, who has had a number of spats with Vettel, will leave.Lotus' Kimi Raikkonen is considered favourite for his race seat though Toro Rosso drivers Jean-Eric Vergne and Daniel Ricciardo are also in the running. Stay with Wheels24 for the 2013 F1 season – fresh reports every day.