Thinking more about this:
1. Might be a decent short-term deal since there are no options for us this transfer market. If he comes for less than 25m and on a 3 year deal then it might be worth it as long as Valverde doesn't try to stick him in a 4-4-2 lineup. We should move away from that and we have no use for Vidal in the league. He should have a high usage in the CL in a possible 4-2-3-1 with Busquets. He can there play as B2B MF to the best of his abilities while we can have a dynamic lineup with Coutinho, Messi, Suarez and Malcom/Dembele.
For the league and copa campaign he should exclusively come off the bench. We need to give Arthur and other players minutes there. He will obviously start from time to time, but if Valverde tries to shove him into the lineup all the time with his knee problems and age, he'll be exhausted when we actually might need him, in the one competition he's yet to win, and faced losses against Real Madrid and Cristiano Ronaldo who he hates with passion. If he can play with the determination and fire needed there we might have something very useful for a couple of years.
But next year we definitely need to revisit the transfer market again. If a player like Pogba becomes available and we also get de Jong and we still need that kind of midfield immediate quality Vidal should not be a hindrance to any such deal. Can understand this now as we have few options and he could present some good value.
But please Valverde, please...don't continue with your shitty 4-4-2 and don't try to force Vidal, Rakitic and Busquets in together. Play a mix of a 4-3-3 and 4-2-3-1 with Vidal as the B2B MF alongside Busquets when needed in the League and CL.
Also if Vidal is coming now, we should easily try to sell all of Gomes, Denis, Rafinha. Denis in particular have no value at all now. Move on and free up space in the squad for Valverde so he can use some Barca B midfielders for easier La Liga games too if he wishes so, and is not forced so include our 1st team players which he seem to prioritize if we have a full squad of 25 players.
Do I like this deal? For less than 25m he might have some value as long as this doesn't mean we won't sign de Jong and/or a possible established midfielder that becomes available on the market next summer, and as long as this doesn't mean the 4-4-2 is here to stay and that Vidal is gonna get physically drained on pointless games where Arthur, Alena, Rakitic and Coutinho can do the job.
And hire someone to keep his partying and drinking habits in check. I don't think Vidal will become an Arda like freeloader simply because he's going to hold grudges against so many people he would want to prove wrong, but it could easily go off the rails if they don't have someone keeping an eye on him. He lacks the discipline to be able to take care after himself and his family and friends don't seem to give a shit about his bad habits either, or are unable to do anything about it.
Anyway, this is my more reasonable take on this after the hysteria settled down and I got some time to think about it.