Despite making the podium, Kimi Raikkonen was far from happy after Sunday's Monaco Grand Prix.
"Feelings were hot in the Ferrari garage," Toni Vilander, a broadcaster for Finland's C More and a personal friend of Raikkonen's, said.
"Kimi's engineer was very upset and Kimi was obviously angry. Maybe things went on that we don't know about."
The suggestion - although Ferrari denies it - was that the team deliberately engineered the situation so that Vettel scored maximum points ahead of the Finn.
"We do not give team orders," team boss Maurizio Arrivabene is quoted as saying by Iltalehti.
"They had permission to fight with each other, and they did so."
Vettel agreed with his boss that Ferrari's pit strategy simply happened to work out in his favour.
"We are racing, we get along well, I can understand that Kimi's upset," said the German.
However, from Mercedes's perspective, Monaco was perhaps the turning point of 2017. The former champions struggled all weekend and now Ferrari might have made a clear move to install Vettel as 'number one'.
"It's difficult for the leader here to be passed - unless the team decides to favour one car," Hamilton, now 25 points behind Vettel after six races, said.
"It is clear to me that Ferrari has chosen a number one driver."
The season continues in Canada in two weeks' time.