Vettel retired at the end of last year after a 15-year career which yielded four consecutive world championships for Red Bull from 2010 until 2013.
He ended his run in F1, which also included a long stint at Ferrari, with two years in the midfield at Aston Martin, where he failed to find success apart from a solitary podium in 2021 at Baku.
But just as Aston found a huge leap in performance for 2023, Vettel is no longer around to reap the rewards of his two seasons of trying to help the ambitious Lawrence Stroll-led outfit get to the front.
According to team boss Krack, Vettel can take some credit in the team's current form, which has included three consecutive podiums for his replacement Fernando Alonso.
"I think he has his merits in where the car is today," Krack said. "Because we had many, many meetings last year where he gave us a hint: 'Do this or do that or do not do this with the new car.'
"So, I think he has his merits in here."
When asked if Vettel retired too early, Krack pointed out the 35-year-old had carefully thought through his decision: "Well, if you've retired too early or too late, that is something you have to ask him.
"And then we have to respect the decision that he took. He reflected for a long time before he made that decision and when he made it and if we have to move on, he has to move on.
"And if it's a shame for him, yes or no, this I think you should ask him."
When the same question was put to Alonso, the mischievous Spaniard grinned: "I don't know if he could have made the Aston Martin that fast, so you never know!"
Alonso currently sits third in the championship after his hat-trick of third places in Bahrain, Saudi Arabia and Australia, with team-mate Lance Stroll in sixth.
The Silverstone team occupies second place in the constructors' championship after three races, leading Mercedes by nine points.
