Part of the issue might be that there are two separate, similarly-named magic items: Dyrr's Impervious Vestment (from CArc), and Impervious Vestment (from Magic Item Compendium). Dyrr's is much more expensive and has a higher AC granted, but they both have the Blade Barrier effect. It's possible that the MiC Impervious Vestment was intended to be a replacement or update of Dyrr's. For what it's worth, I'd rule that's the case, but your DM may not agree. The MiC version does list the Torso slot.
Assuming that you're using the CArc version only, it's a bit complicated. The description of an item called a Vestment describes it as a robe; but (from the SRD, Magic Items on the Body), it could either be the Torso or Robe slot:
Pretty clearly "ask your DM" territory there. Personally I'd say that the fluff description calling it a robe is the more operative thing than the item name, but I can absolutely see somebody else ruling otherwise.One vest, vestment, or shirt on the torso
One robe or suit of armor on the body (over a vest, vestment, or shirt)