Don't get me wrong, I'm not blaming anyone for anything, I just wanted to provide an answer to your question. The problem is, Bungie's design has Vanguard and Crucible marks as mutually exclusive currencies. If they can be converted, it renders the dual currency system redundant. You might as well just have one type of mark that everyone accepts, but the intention is to incentivise both the PvE and PvP arena.