I would currently play him in majority of Camp Nou La liga matches against midtable and bottom table teams.
Against those opponents, even Bartra and similar players always looked like world class players.
(Since those opponents rarely attack, and when they DO attack, those attacks are like: 5 Barca's players vs 2 opponents).
In short:
-- against stronger teams, Roberto at least can defend to some extent. But our attack will be dead with Roberto.
-- against weaker teams who defend all the time, those teams won't even create any chances and Vidal won't have too defend. In those circumstances when we have to attack and create actions, Vidal is Messi compared with Roberto in attacking actions as a RB.
So, against better teams, we have to see whether Vidal can improve in defending.
Against weaker teams, he currently offers more than Roberto.
Something like Mascherano vs Roberto/other players as a Cdm.
Masch is better in defense, but offers nothing in attack and in build up play.
So, against any team who isn't too strong, Roberto and similar players (who are good in attack and build up, but weaker than Masch in defense) are way better options than a defensive-minded Cdm Mascherano.
Against crappy teams, with Vidal as a RB, our Rcm will be better, and Messi will maybe stay on a RW, since he will finally have someone to whom he can pass the ball (Rcm who doesn't need to babysit for a RB+Vidal).