While team-mates at Mercedes Hamilton and Rosberg spent three years battling exclusively for the drivers' title, with Rosberg finally getting the better of Hamilton in 2016.
After Ferrari driver Vettel won the 2017 season-opening Australian Grand Prix, Hamilton was asked if he faced a tougher test from the four-time champion than the one he got alongside Rosberg.
He replied: "Definitely, definitely. I think this year we're seeing the best drivers at the top.
"Obviously it would be great to have Fernando [Alonso] up there, but it doesn't look like that's going to happen anytime soon.
"At least we have a great driver in Sebastian with his four titles, [who had] an incredible surge into Formula 1, continues to be a force and will continue to be for many years to come.
"I'm really grateful to have that fight with him, against Ferrari. It's great."
Hamilton's relationship with Rosberg became fractious at times, with their on-track battles resulting in crashes on more than one occasion.
The Briton even said during pre-season testing that he was enjoying working with new team-mate Valtteri Bottas more than any other because there were no "mind games".
Asked to compare his relationship with Vettel to that of Rosberg, Hamilton said he would not be drawn into such comments.
Read the full article on eurosport.co.uk: Hamilton: Vettel battle tougher than Rosberg