I think he'll be improved at Aston Martin without the pressure of Ferrari, but I think peak Vettel of the 2010s will go unmatched. I'd argue peak Vettel was, to a degree, peak F1 for that era. I've said before that he could make those cars dance like no-one else, not even Hamilton. I'd argue had Ricciardo joined in 2013 then Vettel would have beaten him, though it would have been closer than Webber was at that stage.
I'm not saying he was the best overall at that time (Alonso coming close in a shed was more impressive over a season) but it would have been a joy to watch him at work, if it wasn't for the boredom of seeing him win so regularly.
Where he's failed is that you can't do that with the buses they have had since and while he's been fast, he can't operate at the same level as he once did. He's still better than most (including Perez in my opinion) but he'll never get back to his peak and certainly not with these hulking cars.
Aston Martin won't ever be serious contenders whilst they're picking Stroll over another driver* (last weekend showed that, turning a win into a third place), but Vettel has been a winner at two different top tier teams and will help shape them going forward. He might not win at Aston Martin, but I think he'll go a long way to helping them to lay the foundations for the team in the long term.
* For the record, I too think Stroll is better than he's usually given credit for, but he's not and never will be a driver fitting of a top 3 team. He's good enough for F1 in the current era, but not among a team that apparently has it's sight set on winning races. He's difficult to like as a fan though, particularly when he was at Williams. Better have him there than the team ceasing to exist though.