The source of the rumour was undoubtedly Marko.
"I don't know which sources Helmut has, or doesn't have, but they seem to vary, let's say," Vettel said in Hungary.
Lauda, the team chairman at Mercedes, categorically denied he is out fishing for a potential new teammate for Nico Rosberg.
"I know absolutely for sure and clearly that we have not made any offer to Sebastian," the triple world champion is quoted by Speed Week.
Lauda, 65, accuses Marko - the Red Bull figure most directly in charge of developing, signing and re-signing drivers - of playing contract poker ahead of negotiations with Vettel over his services for 2016 and beyond.
"What is Helmut doing?" Lauda asked rhetorically. "He is closing Sebastian's doors, by forcing people to make these public denials.
"But I am sure Sebastian will eventually figure it out and ask him: 'What are you doing?'"