Vettel announced, prior to the Hungarian Grand Prix at the end of July, that he intends to retire from Formula 1 at the end of the 2022 season.
The German made his debut in the sport as a substitute driver for BMW Sauber in 2007, and went on to win four consecutive World Championships with Red Bull after earning his stripes at Toro Rosso.
In 2021, Vettel signed for Aston Martin and has since helped them both on and off the track as they aim to build a successful F1 outfit.
Asked whether the 35-year-old's retirement announcement changed his attitude at all, McCullough told the media: "He is the ultimate professional really, so I would say no to that.
"Maybe he is a bit more relaxed in himself but I wouldn't see that in the engineering office or the way he drives the car.
"Sebastian is a bit of a machine from the work side of things."
On working with the former Ferrari driver, the performance director added: "Sebastian has been great fun to work with for the last 18 months or so, very thoughtful, very strong on the engineering side.
"He has brought a lot to the team in how we operate and how we look at things.
"A very inquisitive mind, great fun to be with but he also understands how to lift the team from a morale side of things really well.
"It has been an utter pleasure and I'm looking forward to the final races with him."