Vettel pipped Fernando Alonso by three points, having entered the second half of the season with a forty point deficit to recover.
"In many respects this is his best Championship because he has had to work so hard for it. His mental strength has always been impressive but this year he has again stepped it up another level," said Horner.
"He’s never allowed himself to get distracted no matter what you know, no matter whether people have been trying to get under his skin or not, he has never allowed his focus to wane. It almost seemed that the more pressure he was been under, the better he delivered and he knew, coming out of the summer break 39 points behind, he knew he had to start delivering and I think he really stepped up a gear, especially as we got to the Asian races."
"Combined with some upgrades we bought to the car that bought some more performance to the car as well, he made he most out of them and put a string of four consecutive race wins together, which were hugely impressive. In addition, the way he composed himself in Abu Dhabi and his drive back through the field was again massively impressive and the mental strength on Sunday, a lot of lesser drivers would have wilted under that pressure."
Horner also praised the members of Red Bull Racing, who ensured that the Milton Keynes based team won a third successive Constructors Championship.
"They have always worked hard, but this year has by far been our toughest challenge, it has been the hardest championship – Constructors’ and Drivers’, as we have really had to fight our way back into them and that it what has made it the most gratifying in many respects."
"There are so many heroic stories this year of people going beyond what they are asked of to deliver, to get performance to the car, to get parts to the circuit, to get development into the wind tunnel, to hit all of these targets and what you see on a Sunday afternoon is a very small percentage of what is involved in a Grand Prix Team and this Championship, a triple Championship, would never ever have been imaginable without the commitment and dedication of the staff."