M203 Grenadier?
http://www.blackhawkindustries.com/product_detail.asp?product_id=1185&d=
A more modular form of the Tac Vest, in the Blackhawk configuration?
A more modular form of the Tac Vest in general?
http://www.wheelersonline.com/detail.asp?product_id=1016
Even with a pure, modular vest, there's the issue of attaching a belt, to lash more kit onto. Now, most belts are adjustable at the centre, by the Fastex/slide buckle. Most vests are adjustable on the sides. Adjusting on the fly, 82 pattern webbing or Tac Vest alone, can be done easily enough, but is really difficult to do with a vest and belt combination.