The Spaniard will head into this weekend's US Grand Prix 10 points behind Vettel in second place.
"It's the fifth time I arrive in the last races fighting for the title," BBC Sport quotes Alonso as saying. "We have some chances, [but] we need some help from Red Bull."
However, Alonso refuses to give up hope of winning his third world championship title this year.
"I am very confident," said the 31-year-old. "I trust my team and I trust myself and we are honest with ourselves. We know we don't have the quickest package out there, and we qualify on average this year sixth or seventh.
"So if we qualify in sixth or seventh people will say you wave bye-bye to championship but it is our normal position.
"That is not our strong point, our point is to score more points than the others on Sunday and I am sure we will do that in the next two races."
Vettel can win the championship this weekend if he scores 15 more points than Alonso at the US Grand Prix.