Great vault Mythic question

  1. Do I have to complete different Mythic+ dungeons to get multiple great vault choices?

  2. Does completing the same dungeon at different keys count towards the GV requirement?

  3. What about completing the same dungeon at the same key multiple times?

No. If you run +7 tirna scithe 4 times, your vault will have 2 rewards at 213 next week.

Yes.

This seems like a rephrasing of your first question, in which case see the first answer.

To 3. - Doing the same dungeon for all of them doesnt restrict the loot table to just that dungeon.