I think another reason why it would make sense for Mercedes to get Vettel is he would make a good mentor for Mick Shumacher.
And yeah, it isn't a difficult job for an experienced and skilled driver to outscore Stroll. However that driver has a really big responsibility to keep the team as high as possible in the standings also.
Last season, Alonso collected 206 points while Stroll had only 74 points... If Stroll was better Aston Martin could have overtaken McLaren too.
Obviously there are some things even engineers can't handle, no matter how you design the car, if the engine is not as powerful for example, or if the ERS is not as powerful, basically there are so many things that they could, but stuff like the cars shape and all could be worked into it, stolen basically from Newey and put into Aston Martin. Vettel still did as well as he could, and I think he wasn't really caring about it, he could have gone to any other team, he just decided to retire himself there for a bit more last ounce of money.