However, one Italian journalist says it was Lauda himself who recently confided to friends that Mercedes has inked a 'pre-agreement' with the German.
"I have a lot of respect for Niki," Ferrari chief Maurizio Arrivabene told Auto Bild, "but if you see all of his comments on paper, some are right and some are not."
In conversation with the Finnish broadcaster C More, Lauda slammed the rumours about a Mercedes switch for Vettel.
"It's nonsense, and it drives me crazy," the Mercedes team chairman is quoted by Spain's Marca.
"Ferrari is competitive, he has all the money there, why would he leave?" Lauda said.
And fellow Mercedes chief Toto Wolff also played down claims the German team could be looking to shake up its driver lineup.
"It's all market rumours," he said. "At the moment we are happy with Valtteri and Lewis and our goal is to keep the status quo."
But even Vettel himself is not committing to Ferrari beyond 2017.
"For the first time in a while I am in a situation where I don't know what will happen next year," he said. "But I'm not worried.
"I am focusing on what is happening at the moment and how we can further improve our car for the fight against Mercedes."
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