Via feat comparison, Base Zoro is likely >~ Gear 2nd Luffy at this point, so about Hancock/Jimbei level. As said before, we've yet to see him use his full power, such as Asura or even higher, though I imagine it's at least in the same league as Law and Gear 3rd Luffy. Iirc, Oda once stated in an interview back when the Skypiea Arc was current that he tended to have Zoro be nearly equal to Luffy at least in most arcs, so Asura Zoro being around G3 Luffy at least seems to work fine for now. If he has a power up beyond that, it wouldn't be a stretch to place it on the same tier as G4 Luffy, so Vista tier does seem quite appropriate as far as assumptions/hype goes.
From general assumptions, it also seems natural that we'll see Zoro a lot in the Wano Arc, so that'll be truly the time to gauge him. If the Straw Hats have some kind of conflict with Jack in the current arc, then that'll help gauge him as well, since scaling should place Jack at near Marco's level.