The Ferrari driver trails Vettel in the Formula 1 drivers' standings by 29 points after his disappointing seventh place in the Monaco Grand Prix.
But Alonso, who has scored 78 points this season compared to 76 at the same point last year but is in a weaker position in the championship, has no doubts the Red Bull driver is due some bad weekends.
"We have two points more than last year and we were leading the championship last year [before Canada]," said Alonso.
"So in a way, we are doing better than last year.
"What is a little bit out at the moment are the points that Sebastian has scored in these six races.
"Unlucky races will come to everyone and will come to him as well.
"It came to Kimi [Raikkonen] in Monaco with the accident and we are only five points behind him.
"It will come to Sebastian and we need to maximise."
The Spaniard believes that he has been less fortunate that Vettel already this year.
He pointed to Vettel surviving a brush with Lewis Hamilton's Mercedes at the start of the Spanish GP as an example of how fortune has been on the Red Bull driver's side.
"The team has been very unlucky with the situations we have faced this year," said Alonso.
"We saw many people touch in the first corner, last time it was Barcelona where Hamilton and Vettel touched at Turn 1 and nothing happened.
"But I touched [the back of Vettel's car] in Malaysia and we were out of the race, which was very unlucky.
"The DRS does not open sometimes, but not closing never happens so we were a bit unlucky [in Bahrain] too."
Given the ground lost already this season, Alonso has set his sights on a run of strong finishes between now and the August break, which follows July's Hungarian Grand Prix.
"In Monaco, we didn't score so many points so we need four or five consecutive races on the podium or close to the podium to recover some of the points we have missed.
"This weekend, we must score good points."
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