It's a good move for both parties. In Sainz they get a fast driver that looks the part, is a sponsors dream and works hard with the engineers to understand the car. He also speaks fluent Italian. He's fast enough to push LeClerc, he's likely to beat him some of the time. For Sainz, he gets a chance at a top team and he will think he can beat LeClerc, although he may not be allowed to with Ferrari.
Ricciardo gets into a car that is on the ascendancy, where Renault have faltered and seem to lack direction. He will have wanted the Ferrari drive however they have backed LeClerc and don't want to pay two high earning drivers. Ricciardo will be pushed by Norris who continues to prove himself as competent and likeable. Norris will learn from him as well.
For Vettel, I think that the Covid outcome which has drastically shortened this season (if they race at all) and pushed back the rule changes to 2022, have combined for him to know that the red cars are not an improvement on last season and so he is likely to be unable to challenge again until 2022, when he would be out of contract anyway. Usually a driver would be smart enough to have a seat ready before breaking a contract but in this case, I think he might actually retire or at least take a sabbatical until 2022. He has repeatedly said that he doesn't need the money and is enjoying extending time with family away from racing.
The only think that might keep him in a car is Mercedes if they choose to swap out Bottas. That would be a big gamble for all parties but also one that sells newspapers and so you never know.