On moves like Volley, which give you an extra attack, or Hack, which does a similar thing. Does this count as "Multiple Attackers" being effectively Damage Advantage, or is it two or three fully separate attacks, dealing 1d8 damage, and then dealing another 1d8 damage?

If a player rolls a 1 for Vigor at character creation, do they not get to interact with the game's main mechanic, or can they "restore" it up to 6 (which I assume is the maximum max) ?

For skills like Volley and Hack, you roll damage twice rather than have damage advantage. So you roll damage twice and take both totals rather than choosing the higher total.

If an adventurer starts with 1 Vigour, then their max Vigour is 1, meaning they’re more at risk of permanent death. The Warden may choose to allow a re-roll for characters with 1 Vigour, or it could become a more challenging adventure. Wardens can also choose to include ways to increase Vigour, through a relic or as a reward for slaying a powerful boss.