I'd guess that the reason why the room of requirement didn't become a passage way for the Death eaters to get into the school by was because it is, effectively, just a room. A room that is subject to what the person wanting it requires it for and which will become what the person wants. It is unplottable, but as far as becoming a vortex - such as the gate between Kings Cross and platform 9 3/4 - it isn't that magical in its nature.