Shielding is the only mechanic left that is keeping True Damage in check. If you add anything into the game that enables champions (such as Vayne) to bypass shielding, they're going to abuse that and become broken against champions that rely on shields (because they can bypass the focal point of the shielding champion's kit).
Some champion's only Utility in their kits (Riven for example) is a shield. If Riot creates an item that can bypass that, those champions are going to need buffed to compensate and probably will become too strong as a result. I believe this in part, is a reason why such an item doesn't exist. It's not a good answer to shields being strong as some champions can abuse an item like that and other champions will have the most essential parts of their kits becoming worthless with a single item.
A better answer from a balancing perspective, to keep shields in check, would be to nerf shield stacking by giving it diminishing gains (such as locket has in place currently) or else make it so shields do not stack and the stronger shield replaces the one on a champion.