Following last season's Belgian Grand Prix, a furious Whitmarsh described Vettel as "the crash kid" after he took Jenson Button out of the race by spearing into the McLaren.
It was among a number of high-profile crashes that blotted Vettel's copybook en route to claiming his maiden championship at the death in Abu Dhabi.
This season, however, Vettel has been almost perfect, with the only surprise on Sunday being that he could only manage third in the Japanese Grand Prix.
It was more than enough, though, for the 24-year-old to secure his place as the youngest back-to-back and two-time title winner.
Asked as to how far Vettel has come on since Whitmarsh's crash-kid comment, Horner said: "He's had to eat those words this year, hasn't he?
"Sebastian has just benefited from more experience and the level that he has operated at has been phenomenal this year. He really has raised the bar.
"He's come into the year as the reigning world champion and he's performed like a true world champion.
"He really has risen to the challenge, driven absolutely impeccably this year, and it's been a pleasure to see."
As far as Horner is concerned, Vettel learned the lessons from last year and eradicated all the niggles.
"There were little things last year, but there were three race wins that were definitely taken away from him through mechanical issues," added Horner.
"Whereas this year the level he has demonstrated, not just on a Saturday, but on a Sunday as well...
"If you think back to the race in Barcelona where he was under acute pressure, and then in Monaco where he was under monumental pressure. That was an absolute standout drive for me this year.
"Then in Italy, you think of his passing move on (Fernando) Alonso to win the race in very dominant fashion there.
"He's been in absolutely fantastic form, and he's maintained a continually high level for 12 months now.
"He thoroughly deserves this championship victory, which he has dominated since the first race in Melbourne.
"It's the culmination of a lot of hard work and teamwork, and he can be very proud of what he's achieved."
For his part, Whitmarsh was happy to hold his hands up and acknowledge Vettel's performance this season.
"I'll eat them," said Whitmarsh of his words on Vettel last year.
However, he added: "At the time he had a car he should have easily won the title with, and quite early, but it went down to the wire, and ultimately he was quite lucky to win it.
"The point at which I said that, it may have been taken slightly out of context, but I seem to recall it was in a diatribe from me following an incident at Spa where he had taken Jenson out.
"But I have said, and I'll say again, I think Sebastian has driven fantastically this year.
"With the help of Adrian (Newey, technical director) he deserves this year's championship. He has been faultless really, which is great for him.
"Now we are going to try to beat him in the next four races, and then we'll try to make sure we come out next year with a better car than Red Bull and try to win more races and championships."