Considering that you can build reputation and earn marks for Vanguard gear by playing PvE. Considering that you can do the same in PvP to buy Crucible gear. Considering, while championing for a faction, you can build reputation by playing either in PvE or PvP. It seems to me we should be able to buy FWC, DO and NM gear with Vanguard marks. Vanguard gear is strictly acquired through PvE, Crucible gear strictly through PvP, and faction gear is acquired in both. Seems logical since reputation can be built by playing either PvE or PvP content.