Ricciardo took the plunge and left Red Bull after a decade under the team's guidance, half of which was in the senior squad, and signed with Renault for the 2019 and 2020 seasons, citing the need for a new challenge.
Vettel is now into his fifth season at Ferrari, but his 2019 has come with much frustration as the Scuderia have failed to keep pace with Mercedes, leaving the German facing another year where defeat to Lewis Hamilton looks likely.
It has prompted wild reports that Vettel could even retire from F1 at the end of the year – with Ricciardo mentioned as a potential replacement.
And Ricciardo says he can empathise with any negative thoughts coming to his former team-mate.
"Maybe he's experiencing something like what I felt a year ago: frustrated and vulnerable," Ricciardo told Corriere della Sera.
"But he remains a fast driver and can always win a title. The only doubt: does he still know how to keep up top form across a season?"
Ricciardo and Vettel were colleagues at Red Bull in the 2014 campaign, which saw the latter's run of championship success come to an end as Mercedes' dominance in the V6 Hybrid era began.
"In some situations he had a loss of motivation, seeing that he could not impose himself," Ricciardo explained.
GPFans is a multi-platform, multi-language brand dedicated to Formula One coverage. We bring you all the ins and outs of the sport, 24/7, everything from up-to-the-minute news and features to the latest viral stories and clips.
We believe that a new generation of exciting, outspoken drivers will make F1 more popular than ever before, and we want to give our users access to as much of their heroes as possible, on and off the track. From Lewis Hamilton to Max Verstappen, Daniel Ricciardo to Sebastian Vettel, we provide in-depth analysis of every every Grand Prix in the season, from Australia to Abu Dhabi.
With Formula One under the new ownership of Liberty Media, how the sport is being covered is evolving, and GPFans will look to be at the heart of this progression into new media, as one of the fastest-growing sites covering the king of motorsports.