It kinds of depends on what Vettel wants out of F1. If he wants to win championships the Mercedes car is the best but the Ferrari isn't far behind. Given another year the Ferrari could be the dominant car so he has a good chance at having the best car next year. The problem is his teammate, he also wants the championship and it seems he has the edge on Vettel at the moment.
I don't think there's anywhere else for him to go, Red bull won't want him, Maybe McLaren can make a come back but those are gambles.
If he just wants to be a racer for whatever amount of time is left in his career he's as well off staying with Ferrari, rake in the cash, he might get a championship out of it or he could help LeClerk beat Mercedes.
I don't know that Ferrari have a better option than Vettel at the moment, they are already taking the chance on LeClerk, maybe if he works out they'll take another gamble.