Vettel, victor of two of the opening four races of the season, leads the title race on 86 points, 13 ahead of three-time champion Hamilton.
The Spanish Grand Prix is live on NBCSN and the NBC Sports app from 7am ET on Sunday.
Vettel recovered from engine problems in the third practice session to finish behind Hamilton - though he leads the championship by 13 points - with Valtteri Bottas and Kimi Raikkonen on the second row.
Stroll had also been better than the 17 th time at the end of three sessions of free practice regulations.
"It's been an awesome job by the team so I am just super proud that we can get back up there - a better weekend for us so far", he said. We'll work on it.
"We have to wait and see what the updates will bring", Verstappen said. I felt yesterday that it was in the auto, but I just couldn't get to it.
"There are two top teams fighting for both championships and I expect Red Bull will also eventually join the club", Mercedes executive director Toto Wolff said.
The 35-year-old Spaniard, yet to finish a grand prix this season and who failed to even start in Russian Federation after an engine issue on the formation lap, suffered an oil leak before grinding to a miserable halt in front of his home fans.More news: Macron's parliamentary candidates gather in Paris
More news: Russian Journalists Appeal to US Media's Professionalism
More news: Senate intel committee subpoenas Michael Flynn for Russian Federation info
"We are obviously in a much better position, but it is still early", Vettel said. "Overall, they have been the dominant team and it's hard to break that".
Reigning Spanish Grand Prix champion, Red Bull's, Max Verstappen comes to the Circuit de Barcelona-Catalunya with his winning ambitions too, having run to victory in Y 2016 his only F1 win to date. "I thought I had a lot of speed but I never really managed to make a good lap out of it".
Bottas described his performance as "not good enough", but it was enough to confine the second Ferrari of Kimi Raikkonen to fourth.
"Quite amusing at one point when one vehicle on one side of the garage was one or two mechanics, while the other one was completely crowded like bees hovering around the auto so I think great effort so big thanks".
After the checkered flag, Ferrari tweeted that there was "no real engine issue" but that it would be replacing some components as a precaution.
Hamilton has only won one race this season to Vettel's two and a win on Sunday would help close his 13-point gap to the Ferrari driver in the standings.
"It's nothing new, tomorrow we try again". The vehicle is very nervous this weekend, and we were unable to put all the pieces of the puzzle together, "he said. The auto has been working pretty well, I'm happy with that, but it's a bit disappointing doing small mistakes here and there and never really putting a good lap in".