We have seen from Schumacher that retired at first but was convinced by them to come and race for Mercedes because they promised him to build the car together and he did build a great car that won many Championships after he lost his "life" and for Vettel can be easier to bring him back even if he retires if they give him a winning car.
I believe any driver at whatever age would be happy to jump in and drive such car which would revive the best memories of F1 for Vettel and I think he can't say no to such offer.
If he has two sides now and one of those sides wants to stay in F1 then maybe he could surprise all of us by signing for Alpine. He can still race for a few more years normally but it is up to him after being 35 years old. Raikkonen raced even after being 40 and Hamilton also might reach nearly that age before quitting F1.
By the way I have seen a new update that Vettel won't be a part of this test by Alpine. But if there are really some ongoing talks between Alpine and Vettel it feels really exciting. It is always good to continue to watch one of the most valuable drivers in F1 even if he doesn't have a too competitive car.