I'll second the comment that wearing Tac-Vests while crewing a vehicle doesn't work too well. Tried attaching my pouches to my IBA and constantly got hung up in the turret and hatches. Worked much better to have a "clean" IBA/Ballistic vest to wear in the vehicle, then don a separate LBV for patrols/dismounted work.
We get alot of guys in the Marines and entire units that are making it SOP to attach their MOLLE pouches directly to the IBA. For line guys it's not so bad, or even for the scouts in LAR that ride in the back of the 25's, but for crewmen...forget about it.
Another serious disadvantage to doing away with a separate LBV and mounting everything on your IBA is operating in a MOPP environment. IBA is worn under the MOPP suit (at least in the USMC) to avoid contamination of the vest. It's alot cheaper and logistically efficient to swap out a contaminated LBV than it is a ballistic vest. If your IBA is under your MOPP suit, you need a dedicated LBV to carry your gear.