Leclerc has replaced Kimi Raikkonen at Ferrari for the 2019 campaign after just one year with Sauber, while Vettel is preparing to bounce back from a mixed 2018 in which he fought for the title but made several high-profile errors.
Button has joined Sky Sports F1 as a full-time pundit for this season and, speaking at a day of promotional filming for the channel, the 2009 world champion compared to Vettel's situation to when Daniel Ricciardo arrived at Red Bull in 2014.
" It's a very different atmosphere for Sebastian at Ferrari than it was at Red Bull, "Button customs Sky Sports News.
" Maybe there's more pressure. We're going to see how it copes this year. It's going to be a big year for Sebastian.When Daniel came into Red Bull he found it difficult
"Daniel puts him under pressure and actually outperformed him.
" I think if Charles can do that this year it's a tough decision of where Sebastian is going to go for the future and wh that he's going to do. "
Button said Vettel, a four-time world champion, is an" unbelievably skilled driver and super-fast ".
However, the former McLaren and Honda driver said:" If your mind Not in the right place, then forget it.
"Charles seems like a happy-go-luck guy, really relaxed, and I think the team are really going to take him. And that sometimes can hurt the other driver. "
Vettel led the 2018 championship before mid-distance but cost himself by crashing out of the lead of the German Grand Prix and spinning early in three other races
Button's fellow Sky pundit Nico Rosberg, the 2016 world champion, agreed that Vettel is under pressure against Leclerc.
Speaking at the same event, Rosberg told Sky Sports News: "He needs to improve and the team needs to.
"We've made some major changes, we've got rid of [team principal Maurizio] Arrivabene and put in [Mattia] Binotto.
" They've made a change in the highest position there. Also Leclerc could also be very, very positive dynamic – but could also be negative, it could also backfire.
"That's going to be awesome to watch." after spending 2018, his second year out of full-time grand prix racing, focusing on his title-winning Super GT campaign in Japan.
He said that also come as targets and important year for his future, as Red Bull has tied up with Honda.
"For Max it's a tricky one because he's always moving forward, he's always looking for something else," said Button. "He jumps in a Red Bull and wins his first race.
" This year is a really important year for him to keep it moving. He has been fighting for the championship or I think he's going to start asking questions where he should be in the future. "