If tanks are indeed forced to use VIT gear instead of STR, adjustments should be made to their damage output, aggro generation and DPS checks from fights that rely on it. Or maybe their DPS stance can work like a Cleric Stance and swap VIT/STR points when it's up.
Maybe have VIT increase pdef and mdef in a sort of exponential way and/or make VIT buff certain cooldowns directly that are currently affected by STR. Dps on tanks shouldn't be as extreme as it is now (mainly on Warriors in dps stance, who can outdo some single target dps with same gear and skill), or they're essentially filling 2 roles. What would stop a party going 6 Warriors and 2 healers (for mainly single target fights, not A2S) if Warriors remain as OP as now? They'd have unkillable dps, diamond cannons instead of glass cannons.