"He is a fantastic driver," Alonso said, "[Vettel had a very, very strong season, dominating especially the second half of the season with many, many wins. Well done to him and the team. Hopefully next year we can make things a little more difficult than this year."
who won the championship in 2005
and was second in 2010 and 2012, said it was not up to him to decide whether a driver's standing in the all-time list is defined by the number of titles he has won.
"Different people have different opinions and we need to respect all of them," he said.
"Many people think Ayrton was the best in history, the best ever. He won only three championships, compared to two people who have more.
"Some people think Michael is the greatest. If you see the numbers he should be the best one because he has 91 victories, seven titles etc.
"There will always be many different opinions and this is the good thing about the sport."
Asked if he thought he could beat Vettel if he had a car to match he Red Bull, Alonso said: "Again, this is a guess. I am not here to guess, I don't have crystal balls.
"Obviously we all feel confidence we can do a good job but this is impossible to say. Today is a day to congratulate him and for him to enjoy the championship.
"It is a day for us to start working even harder because we have a very strong challenge in front of us.
"They [Red Bull] have been very strong, dominating F1 for the last few years so we need to do something better than we did the last four years because it is clear it is not enough. We will start immediately and next year we want to win, so we have to work."
Mercedes driver Lewis Hamilton added: "It is an incredible achievement for such a young individual. He is in a class of his own, and he is on his way to becoming the greatest driver in F1, if not already.
"Big congratulations, I'm really happy for him. I hope in the future we get to race together, but until then we'll keep trying.
"He's just got to enjoy it. All the rest of us are trying to catch up and we'll hopefully be hunting him down in the following years."
The Indian Grand Prix - live on BBC TV, radio and online
Sunday 27 October : Race repeated at 16:00 GMT BBC Red Button. Highlights: 19:00 GMT on BBC Three
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