Vettel called time on his illustrious career at the end of the 2022 season with four Drivers' Championships to his name.
It was a tough year, to say the least, but Aston Martin did recover a bit towards the end of the campaign with Vettel scoring points in four of the last six races of the year.
"I think he ended the year in top form," Krack told Motorsport Magazine. "He tasted blood."
"When things went up in Singapore and Suzuka, you could see his joy. When you heard him on the radio, you immediately knew he was there.
"He was sharp, he was really in the flow. You could tell in the last races that a car that gives him more feedback, more performance, motivates him even more.
"I have to say: hats off! He had a difficult time, often dropped out in Q1, but he never let his head down and always continued to support the team and Lance. That was very exemplary.
"I'm glad he's where he is - but it's a shame he decided to quit. I hope he regrets it."
While he isn't keen on a full-time return, Vettel admitted he would consider making a comeback for a once-off trip in an F1 car around Suzuka and he has been widely tipped to become a team boss sometime in the future.
"I always liked, for some reason, rallying," Vettel said, "but I can see it's a major challenge, because it's so different to what we do in, let's say, classic circuit racing. So I don't know. We'll see."