Ferrari is focused ahead of Vettel's arrival.
It has been suggested that, after winning four titles on the trot, German Vettel could not cope with being comprehensively beaten by team newcomer Ricciardo in 2014.
But Australian Ricciardo is not sure it is quite as simple as that, even though "the team duel probably didn't make Sebastian's decision any harder".
"He didn't have the year he wanted," Ricciardo also told Germany's Sport Bild.
"I know how great his passion is for Ferrari," Ricciardo explained. "Not just with F1 but also the road cars. He is a fan of the brand and its products.
"So I think it would have happened sooner or later."
The fact that it has come sooner for Vettel, however, means he is arriving at Maranello amid great turmoil.
Stefano Domenicali, Luca di Montezemolo and now even Fernando Alonso and Marco Mattiacci have all fled during the course of a disappointing 2014 in red.
The turmoil moved Ferrari's new president Sergio Marchionne to seek to reassure the team's staff in a letter this week that was leaked to Italy's Autosprint.
Message from the big boss
"I understand your disappointment at the end of a season that we would all like to forget," he wrote.
"But, as often happens in life, it is the dark moments that push us into phases of renewal and rebirth, so today we have the opportunity to start a new chapter in the sports history of Ferrari."
Marchionne said Vettel is the right choice to replace Alonso, and not just because "Sebastian and Kimi (Raikkonen) are united by a great friendship in life".
"We all know how important it is at this time to have a healthy team spirit," he added, "coming from people who believe strongly in the project and want to share their commitment, sacrifices and achievements."
Marchionne warned that the road faced by Ferrari beginning in 2015 "will not be short nor easy", but that change "should not be feared".
"Progress is made by those who decide to break habits and choose not to be the result of the past, but one of the causes of the future", he concluded.