Play ROC strikes until you've 125-150 marks. Each rank after 2 gets you a vanguard commendation (plus a free legendary item from the vanguard, could be armor, shards, weapon..etc) , so do lots of bounties. You need 1 commendation for (Vanguard legendary) Chest piece and 1 for helmet, so by the time you're level 4 vanguard reputation you should have all the commendations and marks for all 4 pieces of armors.