Of course, along with the prejudice against all things Flan that characterizes most Oeridian and Suel societies, we must also call attention to the vast resources that the Keoish in general (and the Neheli specifically) have expended over the centuries in suppressing general knowledge of the Occluded Empire and its various manifestations.  Every obvious and well-preserved outpost of Vecna's power within the reach of the Neheli would have been dismantled, purged of whatever taints lingered there, and (using that medieval model again) used as building materials for new structures.  One wonders how many of Dorglast's stones were 'quarried' from those curious, seven-sided basalt monoliths from the edges of the Rushmoors...

Oh, and of course, the Neheli would have kept anything magical that they found in said ruins...for safekeeping with the Silent Ones!  Safekeeping...yup, that's the ticket...safekeeping...