Rather than switching tanks' attack stat to VIT, I think they'll just

-tag Slaying gear as DRG/MNK only going forward
-tag Fending gear as Tank only going forward
-add a small amount of STR to fending gear going forward
-add 2 materia slots to 3.2 fending gear and beyond

With 17 STR/ilvl appropriate VIT/2 slots, 3.2 fending accessories would be a 100% upgrade over current slaying. Depending on how much DPS they want tanks doing, they might lower potency across the board. Alternatively, they might just make the new STR amount their baseline when determining how much DPS a fight will need (though I think they were calculating for STR accessories already in Alexander).