It's not as simple as a straight swap, but obviously I would take the turnover of Vecino out and Eriksen in.
That said, I've always been a big believer of having variety in the midfield. In terms of what they bring to the pitch, Vecino departing would leave us with a void of his type of midfielder.
I'm simplifying greatly here, but I always saw Vecino as kind of a shit SMS/Nainggolan/Vidal type. i.e. a physical and direct goalscoring midfielder who can drive forward with the ball. Indeed, his best game for us was the 3-2 derby against Milan where Spalletti played him as an AM of sorts.
People often compare Vecino to a horse, and this comparison aptly describes both his good and poor qualities. Eriksen definitely scores goals and brings other assets that we lack, such as set piece quality. But if Vecino were to leave, we wouldn't have that physical attacking CM in our squad anymore.
Eriksen actually covers a ton of km's too. He's not a lazy Ozil type. But he really doesn't tackle much at all, and he doesn't have that same horselike, galloping grinta, speed, physicality and aerial threat that Vecino has.
I've been skeptical about Vidal for a number of reasons, such as his age, mohawk, partying habits, and that we have Nainggolan on the books etc. But that's one area where Vidal would be undeniably more useful than Eriksen. He brings that dominant physicality, speed and power to the midfield that Eriksen simply doesn't possess.
That said, all things considered, I would still opt for Eriksen over Vidal. I would also sooner replace Gagliardini with Erisken(/Vidal) than Vecino but whatever.