Long term, I think yes [re: Vettel staying at Ferrari].
Vettel IMO is enjoying something at the moment that is hard to come by: Team-leadership. I'm not sure he truly had that at RedBull or that RedBull is the team that allows that kind of dynamics. He went to Ferrari and this move has allowed him to "grow" into a driver that is not unlike Michael Schumacher: The focused, hard-working and consistent performer who is also in the lime light. I think Hamilton is in a very similar situation now at Mercedes than if he had stayed at McLaren. He has unfolded and become a personality, his own brand, something that would never have happened to this level if he had stayed McLaren.
Ferrari are very happy with Vettel. He is more often than not showing performances where one could think is driving beyond what the car is or should be capable of. Maybe this is a bit the result of Kimi underperforming, but I think Ferrari will try hard not to lose Vettel.
The other factor is that the performance gap between Mercedes and the other teams is shrinking. Engine wise, there's nigh on nothing in it. Then there is also the long term commitment of Mercedes. Lets assume Mercedes may stay only till end of 2020 (http://autoweek.com/article/formula-one ... eyond-2020
). What's after that? So Vettel comes to Mercedes for a 3 year deal (18/19/20). And then what? Go back to Ferrari? I don't think that will happen.
I think Vettel is in it at Ferrari for the long run, may even end his career there and with a WDC winning Ferrari, may even create his own Michael Schumacher-esque legacy along the way. I do think Mercedes is the stronger team (as a whole, considering all the resources they are willing to spend and their recent level of success), but going there will put him against Hamilton and he will be giving up his "leadership" position IMO (even if he ends up beating Lewis).
If Vettel wins the title this year, it will be his greatest achievement and a dream come true - a dream that has eluded Alonso over the entirety of his career at Ferrari. I very much doubt he'll leave, not in the position he is in right now.
And Bottas, who is doing a very formidable job so far, I think maybe one of the reasons Mercedes might just prefer to stick with what works rather than risk a highly competitive environment to the level they had with Hamilton and Rosberg.
All just IMO of course.
@ScottB: Regarding Lewis committing beyond his contract at Mercedes: Why not? The team dynamics have changed to the positive and I think this year, he is having one of the best seasons yet (by battling another team with the team fully united behind him). He is also racking up records and creating a legacy for himself. All his life, he has been competitive and I honestly wouldn't know where he'd go if he were to leave F1. He's marketing gold, but only if he continues to be that sportsperson. Same applies to Federer and all the other big sports personalities. Once they end their careers and retire, how attractive will they remain to their sponsors? Don't hear much about Agassi, Sampras or others anymore.
Having said that, I can see him staying for at least another 3 year extension which will bring him to the end of Mercedes commitment. He's not like i.e. Rosberg who has a family now and is thinking about risk vs family.