Last night I completed the mission Ritual of Sacrifice (level 30) (end of Eris urn quests), and the game stated I'd receive 10 vanguard marks for completing it. I didn't.
The first time I completed the mission I also did it at level 30. This second time I was helping my brother since this mission is not repeatable.
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1. You only receive the reward once per character. 2. If you already earned 100 marks this week (or are holding 200) then you cannot earn anymore