Possible Dark Pact bug/issue?

Probably the best bet right now is conc effect instead of spell echo on single target, or if you find boss fights short enough (so you won't run out of flask charges), use atziri flask. Atziri flask will more than double your leech for times when you haven't been savage hit recently.

Secondly use your wither totem. More dmg is more leech. Though honestly I have not tested if increases to enemy damage taken do scale leech the same way as if you do more damage, but It probably does (i assume path of building probably has tested or would have been called out on it for such a commonly used program).

And of course, just leveling up will give you more damage to scale leech assuming that damage isn't from life, so this problem should fade in time.

I understand dark pact is hard to tell your dps exactly since the tooltip does not consider the base damage added from life, and the more multiplier from using your life, but vaal pact life leech is as straight forward mechanic as it gets. Damage*leech% = life leeched. please do at least the bare minimum math required for these claims! Life drop per activation less than 6% of life? life leech confirmed