I'm trying to reduce the number of vertices on a big architectural nif, and in 3DS MAX my vertex count = 3400 (happy with that) but when I check in Nifscope after I export I get vertex count = 8935...........What the Hell???

Tried removing duplicate and unused vertices with out any change. Could floating meshes / geometry be causing this?

I have same "issue". I think that nif files doesn't share vertices for adjacent polygons. In fact, in your case, you have like the double amount of vertices in the nif file.