Nico Rosberg says he is ready to take on champion Mercedes teammate Lewis Hamilton.
Nico Rosberg won five races last year to Lewis Hamilton's 11 and finished runner-up last season.
Vettel, who has replaced Spaniard Fernando Alonso in the lineup, joins Finland's 2007 world champion Kimi Raikkonen.
Spain's Fernando Alonso willl be driving for Mclaren while Sebastian Vettel will replace him at Ferrari.
Horner said it would take time to get used to seeing Vettel in Ferrari's red.
Red Bull will rely heavily on the Australian as Sebastian Vettel is all set to drive with Ferrari.
Five grands prix's will start an hour earlier this year under recommendations made after Jules Bianchi's crash.