1- Foresight is a direct multiplicative buff on your defense stat after your defense from armor and other buffs (protect) are applied.

2- Raw Intuition had a bug where heals also crit from flank/rear. Yes, Magical attacks do crit.

3- Vulnerability Down is the same effect as any buff that reduces damage. It's just "added" on top of Vengeance counter effect buff. This was because in 2.0 Vengeance had no damage down component. But it works exactly like Rampart or Sentinel.

4- Every buff that increases your damage by a percentage increases your drained health by the same percentage. So Unchained (Unchained doesn't affect IB and SC) + Berserk a massive self heal increase from all your abilities. Vengeance alone is a 50 potency counter based on how many and how often you got hit. Vengeance alone is a loser here. But why not use Vengeance WITH Berserk? It's added damage on top of added damage.