It's really simple, put Blade of Woe and Vampire Feed on different key bindings or stances (weapon seathed/drawn)
As for the why, I'll put it like this
if you want to use blade of woe
1: sneak behing target
2: press synergy button
3: ta-da you stabbed them
If you want to vampire feed
1: sneak behing target
2: make sure you are between 40 and 40.3 cm away from target
3: make sure you are perfectly alinged with the targets elevation
4: make sure it is between 16:55:01 and 16:55:02
5: make sure the stars are aligned
6: make sure the plants are in the right place
7: roll a D1000 and get a 777
8: still only a 50-50 shot that it works and you don't just get Blade of Woe triggering instead
Yes I am aware of the vampires woe addon which helps considerbly, but the fact that an addon is needed to fix this problem just shows how simple it would be to actualyl fix and implment into the game