As far as I'm aware this [i]only[/i] works for vanilla armour because the only engrams you can get vanilla gear from are the postgame screen engrams which are only ever armour. You can tell if it's a vanilla engram from the description. A vanilla engram will read something like "The Cryptarch at the the Tower may be able to decrypt this" whereas a new engram will read "An engram of remarkable potency, the Cryptarch will be able to decrypt this into [specific weapon/armour class]". I should add there is one way to get TDB (but not Pre-TDB) weapons and that's from Eris mission postgame. If anyone knows how to be able to dupe these while still getting engrams (I know you can dupe them by deleting the bounties mid-mission but apparently you can only get the engrams once) that would be amazing.