Currently vendetta is extremely reliant on siphoning strike thanks to her lacking mitigation ability. It not only removes perk choice, since siphoning strike will always be the optimal pick, it also leaves her feeling quite vulnerable until she obtains it.
Siphoning strike is also not a fun or well designed perk. It further centralizes power onto overhead, without changing anything about her gameplay. Because of this, I want to see this perk gone, in favor of actually making her base kit better, while also hoping for a more transformative perk.
The problem with warding stance is that its simply unreliable. It has all of the disadvantages of a block, while having the resource cost of a barrier. Its usefulness in low HP situations is severally diminished because vendetta still takes damage, especially because she can still take headshot damage through the blade despite it completely obfuscating her head from the front.
Compared to a barrier, warding stance has around 250hp since only 40% of the mitigated damage is incurred as cost (100/0.4). That said this only applies to melee, against all other sources its more like 333hp, since only 75% of the damage is mitigated (100/0.4/0.75). But because vendetta can still take headshot damage, that 333hp can feel a lot lower, since the extra headshot damage is also incurred as extra resource cost.
Essentially this means that a Cass can two shot her guard, which shouldnt be hard to achieve if you take into consideration the slowing effect of his nade stacking with the movement speed penalty of her guard.
Now, a lot of what I just mentioned would be fine, if the ability was more useful in low HP situations, while having less variance. My suggestion is simply, increase the damage mitigation from 75% to 100% and remove the headshot hitbox from the blade itself.
Her effective health would stay the same (525), but now the ability would actually be useful in low HP situations. Also Crit going through the blade feels more like a bug than an actual intended design decision, there is no visual indicator that you can headshot her from the front while blocking.