Vettel scored his sixth win in a row at Sunday’s Indian Grand Prix to snap up his fourth consecutive title with three races to spare.
Discontent at seeing Vettel continually top the podium led to fans booing, most notably in Canada and Singapore this season.
But Newey, who helped design title-winning cars for Nigel Mansell, Alain Prost and Ayrton Senna believes there is still more to come.
“Sebastian is not going to get worse, that’s for sure,” he said.
“He’s going to continue to develop, although it’s difficult to see how he can develop relative to this season, but no doubt he will.
“His driving has gone from very talented but slightly raw at times, to incredibly well-rounded now.
“It is difficult to see a chink in his armour.”
The season continues this weekend with the Abu Dhabi Grand Prix.