At the time the four-time world champion was struggling with the new 2014 regulations and was getting beaten regularly by new teammate Daniel Ricciardo which did not help matters.
In a chat with The Mirror, Horner said that Vettel came close to making a decision on his future in F1, whether to quit or not.
"There was a stage last year where he thought about whether he wanted to stop or not — whether he was getting the same level of enjoyment and wanted to continue," Horner tells The Mirror.
"He was unhappy with F1, the previous four years had been so successful in a car he loved driving and suddenly things were very different.
"It was like somebody had taken his toy away and took him a while to get a grips with that.
"So he went through a period of disillusion with F1, not Red Bull but the direction F1 was going in and was a bit outspoken in his views," he added.
Vettel has since joined Ferrari, embarking on possibly an even bigger challenge than his 2014 season at Red Bull.