I don't feel like Pep already made up his mind about this.
I think Stones is a given now, and rightly so, so much potential in him both as a defender and a "footballer".
But Otamendi / Vinny and even Kolarov as center back I don't think anyone is ruled out yet.
Otamendi always very energetic, but still looks like a headless chicken during large parts of the game. Often running out of position to attempt an impossible interception and leave loads of space in behind him. We've seen this a couple of times against Barcelona too.
Vinny I think still the best of the lot, but relying on him is a real problem. You want a partnership you can count on.
I really like Kolarov as an alternative, always calm, great passing and vision, but obviously less defensive ability than the others.