Well, to be honest I don't know how much Vidal has regressed since leaving us. However, they have improved their starting XI from last season and have much more depth also (Sensi, Eriksen and 9golan will be rotational players, that's pretty good for any side).
From Gagliardini to Vidal is an improvement, Candreva to Hakimi and Bastoni to Kolarov also. That's 3 positions that have been improved.
Bastoni is a CB though. Kolarov will alternate with Young mostly and will score some free kicks, but he's hardly an improvement.
I'm not sure if Vidal is an improvement over anyone. Gagliardini is a clown, but Inter already have better players than Vidal in Barella, Sensi, Brozovic and Eriken (though he won't play much).
Hakimi is an excellent signing, though I think Conte's style will stifle him.
I think Inter have a good squad, but Conte will implode this year. He can't take his finger off the detonator, no matter how well his team does.