Longbow of Vengeance Upgrade Shadow Strike Kill Online Vendetta
The Vendetta gear set is infamously hard to complete challenges for, if you wait too long.
In order to finish an orc off with a Shadow Strike, the attack must deal enough damage to bring them to 0 health. If the attack brings them close to dead, but with a very small amount of health remaining, you will perform the cinematic QTE execution, as you've seen yourself. Due to your level, you're now trying to kill level 80 enemies with a level 40 weapon, which makes it very hard to deal enough damage to bypass the QTE.
My suggestion is then to increase damage dealt with your bow as much as possible. Equip Warrior Gems in your bow, armor, and cloak slots, try to only complete this challenge against orcs with a weakness to ranged weapons; a mortal weakness to ranged weapons would also instantly complete the challenge, but mortal weaknesses are rare at level 80.
From my personal experience, a QTE execution is more likely to get triggered if the fight has been particularly long, so try not to drag the fight out too much. Check how much a Shadow Strike lowers the orc's health bar by, and try to bring the orc's health below that point before you go for the kill. If you're worried about triggering the QTE before even attempting a Shadow Strike, regular arrows will let you deal damage from out of the QTE's range.
Once you find an orc with a weakness to ranged attacks, I'd suggest just letting the QTE fail on purpose if you accidentally trigger it. As long as you keep trying, you will eventually get lucky and kill the orc with a Shadow Strike.
Hey if you're still struggling with this challenge do NOT use the default shadow strike. My technique was to replenish elf-shot via draining grunts and then pulling the Online Vendetta target using the Shadow Strike pull upgrade once his health is low enough. It may take several arrow replenishments which is why I have it set to give me arrows when I drain grunts... easier than running around the compound looking for inconveniently placed arrows. Once you've restocked keep pulling him to you via the Shadow Strike kill pull. Worked for me. Hope it works for you.