It has nothing to do with driving, with the French 'Professor' recognising Red Bull's 26-year-old ace needs little assistance in that department, and everything to do with how the German celebrates after winning.
"If I see him I am going to give him the advice to stop celebrating with his finger," Prost said with a grin at the Abu Dhabi Grand Prix, where Vettel is chasing his seventh win in a row and 11th of the season.
Vettel's raised single digit has become a trademark of the German's post-race victory celebrations and a particular annoyance for rivals and others who would rather see someone else win.
In India last weekend, he rang the changes by raising four fingers instead to celebrate becoming the sport's youngest quadruple champion.
Despite his advice, Prost expects the German to be in a position to make the gesture for some time to come.
Next year sees significant changes to the regulations, with a new turbocharged V6 power unit with energy recovery systems, and - doubtless to the dismay of many - the Frenchman expected the new era to favour Vettel as much as the old.
The changes, he said after a presentation of Renault's new V6, would favour the more 'thinking' driver who understood fully the workings of the car, the importance of fuel economy versus raw speed and could adapt his style to changing times.
With so much to contend with, those teams with a clearly defined number one driver - such as Red Bull, who have inexperienced Australian Daniel Ricciardo stepping up alongside Vettel next season - could also have an advantage.
"You want to be focused on one thing at a time, because it is very complicated. At Ferrari with Kimi (Raikkonen) and Fernando (Alonso), you don't know what is going to happen," said the former McLaren, Ferrari and Williams driver.
Raikkonen, the 2007 champion with Ferrari, is re-joining the team alongside double champion Alonso in what will be the sport's sole pairing of champions.
"Vettel has to be the team's number one next season. You don't want to destabilise completely a team, especially with new regulations, by signing another top driver. You don't want to create a problem, what is going to be the advantage?," asked Prost.
The German is the only driver in Formula One history to have won his first four titles consecutively and also the only quadruple champion to have won all four with one team.
Prost did not agree with those who argue that the measure of Vettel's true greatness will only be fully apparent when he has won titles with more than one outfit, as the three other four times champions all did.
"For sure, if he moved to another team and wins with them, it would help. But I have to be realistic, if I was in his position, I would not move," said Prost, a champion with McLaren and Williams.
"I moved (teams) myself for different reasons. Honestly, if (Brazilian triple champion) Ayrton (Senna) had not come into the team, then maybe I could have continued with McLaren forever. We don't know.
"You never move saying 'OK, I want the public to have a good perception of me so I move'. There is no way. It does not work like this. He is only 26, so we will have to see," added the former champion.
Prost recognised that fans wanted to see team mates challenging each other, racing on level terms, but that would not be his position as a driver or team owner.
"Look at 1990 at Ferrari with Nigel (Mansell)," he recalled of a season in which he finished runner-up to McLaren's Senna.
"If we had a clear number one and number two, I would have been world champion. No question."