Life Leech Effects Interactions

Hey I am trying to do some planning for a build and I wondering if anyone knows how some of the special Leech Life effects from passives and items interact . . . specifically what are the interactions between "Vaal Pact", "Couture of Crimson Gilded Vestments" and "Fast Metabolism"

Does"Couture of Crimson Gilded Vestments" and "Fast Metabolism" synergize or does one overwrite the other?

How does "Vaal Pact" + "Fast Metabolism" work?

For example lets say I have 100 max hp and I when I hit a monster I am suppose to get 50 life from my leech life.

Does my leech kick in if I am on full life, to bring my life total up to 150/100hp?

Does my leech only kick in if I am below my full life? Let's say I am at 99/100hp before my leech, will that then bring me up to 149/100hp?

Does your hp cap at your max hp regardless of "Fast Metabolism"? Can total life be higher than your max life? and if so, can I keep stacking leech to increase my total life?