MUGELLO, ITALY - Sebastian Vettel says Red Bull can be satisfied with its start to the 2012 Formula 1 season but needs to do more if it wants to defend its Driver and Constructor titles.Vettel currently leads the 2012 Drivers' standings after his win in Bahrain and Red Bull leads the Constructors' log.Speaking in Mugello, Italy, Vettel said the team had some catching up to do and was working hard on improving performance.'DOMINANCE IS NEVER EASY'"It's always good to win. That's our aim but we still have a lot (of ground) to make up which is why we wanted to put in a lot of kilometres in these tests and to examine things we have already tried."Red Bull won 12 out of 19 races in 2011: Vettel dominated the season for his second successive World title but that supremacy appears to have slipped away.Vettel said: "Dominance is never easy. We've had had four races and one win (this year). We were competitive in the other races but certainly not strong enough to win. Nevertheless it wasn't a disaster."We are satisfied with the state of things but we want more. We are working hard on new parts which will improve the car's performance."Eleven of the 12 F1 teams were in Mugello for the three days of testing which ended on Thursday, May 3. The next race will be in Barcelona, Spain, on May 13.McLARENS DRIVERS MISSINGVettel said the circuit was "perhaps not ideal" for testing because of its many turns. "It makes it difficult to say whether it's going in the right direction. We have tested a lot but more important is then the evaluation and what happens in the week leading up to Barcelona."McLaren drivers Lewis Hamilton and Jenson Button and Lotus driver Kimi Raikkonen did not take part in the sessions on the track in Tuscany.Vettel said: "Whether as a driver you have to take part is something you can discuss. Time on the track is always valuable. This circuit is fantastic, one you hope to drive on when you sit in an F1 car. Unfortunately we don't have a circuit like this in the calendar."