Such have been the calibre of Vettel's performances this season, en route on Sunday to becoming a back-to-back and two-time champion, that Horner has told him he needs to improve in two areas - being able to tell a good joke and getting a decent haircut.
Horner has over-used the word "phenomenal" this season to describe Vettel's achievements - 12 poles, nine wins, four seconds, a third and a fourth, standing on the podium 14 times out of 15.
But it is hard to conjure another superlative when Vettel has been so dominant, and the question now is how the 24-year-old German can possibly improve.
That fact was recognised by Horner who said: "It will be hard for him to better what he's achieved so far this year.
"But knowing Sebastian he is never satisfied and will always be looking to improve. That's what makes him such a strong driver."
Horner then quipped: "He still needs to work on some of his English jokes, and some of his haircuts have been a little bit strange this year, so there is that I suppose."
With little time to celebrate as the Korean Grand Prix is up next on Sunday, Vettel's first outing as double world champion took in a visit on Monday to the global headquarters of one of their sponsors, luxury carmakers Infiniti, in Yokohama.
With the drivers' title secured, retaining the constructors' crown is next on the agenda for Red Bull, and maybe a record or two into the bargain for Vettel.
"It's a phenomenal thing to win the drivers' championship, and of course the constructors' to us has equal importance," added Horner.
"The main target now is to close out that. We've a 130-point lead with four races to go, so we are in a reasonable position.
"Of course there are a few records that are potentially available, particularly to Sebastian.
"I'm sure his main focus is going to be to help the team win the constructors', although I'm sure he's half an eye on maybe matching some of those records."
As an ominous warning to the rest of the F1 field, Red Bull advisor Helmut Marko, the right-hand man to owner Dietrich Mateschitz, does not believe Vettel has yet reached his peak.
"He will improve," Marko told Press Association Sport.
"He will be more mature, he'll be better on the technical side, and there is more speed to come because he is not yet at his limit, his zenith."
Three-times champion Niki Lauda believes Vettel could go on to beat the record of seven titles held by Michael Schumacher.
"If the car, his career and where he drives all continue in the right direction, then yes (it could happen)," said Lauda.
"If you choose the wrong car, or the car you are driving doesn't work, then you can't win because it's always the combination of the two.
"But he certainly, theoretically, can win more than Schumacher if he sits in the right car at the right time. He is certainly capable of it."