DOT damage seem to show capped out on a single stack in the tool tip:
I removed all gear except for the unique axe (Taste of Blood). My passives in total contribute 65% to all global damage which is confirmed on the tool tip.
Upon hitting a target with lvl 0 Vengeance, the tool tip shows
This shows the expected damage from baseline bleed + a single bleed instance multiplied by 1.1 which is supposed to be the 10% dmg increase from a poison stack.
Also, the % DOT dmg increase never increases further from this point suggesting that only ever a single stack of DOT is applied even though I repeatedly hit the target with Vengeance.
Full set of screenshots here: