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Sebastian Vettel says the deterioration of Red Bull and Renault's Formula 1 relationship is "sad", because it overshadows how successful the French manufacturer has been.

Vettel won four consecutive F1 world championships between 2010 and '13 with Renault-powered Red Bulls, but endured a tough '14 season before his move to Ferrari.

While the German has won a pair of races this year with his new team and is the closest challenger to Mercedes, Red Bull has been limited to just two podiums amid Renault's ongoing struggles.

The two parties are set to split at the end of a year in which Red Bull chiefs have regularly taken aim at its power-unit supplier.

"Obviously it's sad to hear," Vettel said.

"I was part of the majority of the partnership and we had very successful years, which unfortunately now gets forgotten very quickly because of the situation."

Vettel highlighted Renault's contribution to Red Bull's blown-exhaust diffuser concept during the team's domination of the latter part of the V8 engine era.

"Renault has done a fantastic job in the past," he said,

"Supplying us a strong engine, supplying us with the latest technique that I think was required to be competitive when we had the era of blown exhausts.

"I think there Renault was probably one of the best ones and most advanced.

"So there's a lot of things that people tend to forget now, that Renault has achieved, but I hope they stay in the sport."

McLaren driver Fernando Alonso also won both his F1 titles with Renault, during its last stint as a factory team in 2005-06.

He hopes the deal for Renault to re-acquire Lotus and restart its works operation is completed.

"It's a very big manufacturer with a lot of history behind Renault in Formula 1, so I think if they stay as an official team or whatever, if they decide this is best for them, I think this is also good for Formula 1," he said.

"Renault and Formula 1 are quite linked together in the history of this sport, so hopefully they stay."