Although Vettel has been dominant in the second half of the 2013 season, taking a record-breaking nine consecutive victories, Horner thinks that the German has not yet reached his peak.
"I don't think he can quite believe what he has achieved," said Horner.
"To have nine consecutive victories in one season, and to have been the only driver since July to have won a grand prix is quite outstanding.
"The manner in which he has won as well has been so dominant. We have really seen him come of age this year, and I have said it before that he will continue to improve.
"We have yet to see the best of him, but I think we are starting to see that now."
Although Vettel has been a key element of Red Bull's astonishing run in the second half of the season, Horner is also eager to emphasise the part the whole team has played in producing such incredible form.
"We have worked closer, harder, and more effectively than any year," he said.
"We are always looking inwardly and Adrian [Newey] and the technical team have done a brilliant job.
"But all of the departments, all of the support, all of the unsung heroes that you don't get to see or hear about, they have all gone that extra yard and that is what has helped us achieve this kind of result."