It's rather simple, really.
You copy&paste the NIF file to another subfolder of your's inside the armor folder, likewise for your textures, but these you've done already anyways and placed them somewhere.
Now instead of modifying the Vanilla armor file or replacing the Vanilla texture file you edit the copied NIF file and point it to the new folder for its textures. (Keep in mind to have the textures' pathes remain relative to the Data folder, that is the string starting with "textures/...", or else, using absolute pathes instead, after distribution only people having exactly the same install location as you will have a working copy of your mod!)
Then you can create a new plugin, or edit an existing one, 'duplicate' the Vanilla item record, change its name and properties, and point it to the copied NIF file.
Last but not least, do with it as you please, place it into the world, into a chest, into a vendor's inventory, whatever floats your boat.