Mercedes secured both titles after being utterly dominant in the opening races of the season, winning all of the first eight races.
Only Max Verstappen away from Mercedes was able to win a race before the summer break.
"At that moment you do not give up, but you realize that you are not in the position where you want to be," Vettel told Autosport.
"You fight with yourself first, because you are too far away to fight with others or fight with Mercedes, that race after race got stronger.
"We first tried to find our shape and when we found it, we realized that we When it turned out that we were unable to reverse everything quickly, it became clear that it would be difficult during the European races."