on pure performance yes, Vettel will be lying if he wasnt regretful about leaving F1, because any wouldve loved to have been in that car. & the what if, just schumacher in 2007, Coulthard in 2009, & theres a reason why F1 drivers never really retired Alonso, Lauda, Prost, Mansell all came back to F1. Button at Le Mans, Both him & Kimi going to Nascar. Barrichello & Grosjean in IndyCar & his performances last season in those last 2 months once it is was asian, American run in. he looked back to his best, he had since before germany 2018. but whether that was because the pressure was off & he was just enjoying his final few races. we may never know
but i think he personally made the correct decision, because he wanted to spend more time with his young family & not miss out on the childhood. which with an ever expanding calendar with testing starting at the end of February & the 23 races later finishing at the end of november. something had to give. but if he had known how good this car would be, he may have stayed. but that the benefit of hindsight