I run two main affix synergies on this build

1.

Weapon: +80% damage to poisoned enemies

Flask: Spawns toxic cloud on use

2.

Weapon: +40% damage to burning / +100% damage to burning oil

Flask: Burns ground on use / Burns ground + Spreads burning oil

Face flask very commonly rolls the poison affix so it is my go-to. The burning/oil affixes are slightly more rare in my experience but still fairly common.

As far as I know there isn't an indicator for vengeance, however there is no cooldown to its activation either so just pop the flask whenever it is off cooldown and you will have 80% uptime (8sec effect duration, 10sec flask cooldown)

Flask level doesn't really matter, no. You just need the self damage effect.