6 Item Vayne is extremely powerful, that's why you feel unstoppable. I don't know how to say it nicely, so I'll just say it bluntly. Your item build is inferior. There's a reason Berserker Greaves, IE, PD, LW, BT and QSS has been the standard ranged AD build for tournament level play since forever. Against a tank with a proper build, the standard build will yield at least twice the damage of your build, if not more. If a tank has 150+ MR your Bloodrazor is basically useless, and PD stacking is superfluous.