rotomthe3rd의 댓글

Ranks 1 & 2 drop from Helya herself, and Rank 3 drops from the World Boss Levantus in Azsuna.

dasbaum의 댓글

The buff can't be clicked away like other buffs
The use of the rune is tracked via quest flag FLAG - Vantus Rune Used

Contrary to the tooltip description: You can use a new rune as long as you did not engage the boss yet which is stated in the buff

Example: You use 반투스 룬: 악몽의 용 by mistake, kill Nythendra and Raid was scheduled to progress on Ursoc next. You can still use 반투스 룬: 우르속 to adjust your buff.

You cannot switch runes after you engaged the boss which is stated in the buff.

Macro to check your state:

/run print("Vantus Rune used?: "..(IsQuestFlaggedCompleted(39695) and "\124cff00ff00Yes\124r" or "\124cffff0000No\124r"))