Just get Surge from Storm Calling to prebuff. It used to be you had to backbar 2h for Momentum to prebuff your damage on nightblade
To make up for that condition the Power Extraction morph debuffs the enemy with Minor Cowardice and gives you a 2nd damage buff in Minor Courage. Resulting in an AoE damage skill with two damage buffs Major Brut/Sorc and Minor Courage that last 30 seconds, a 10 second damage and healing debuff on all enemies hit in Minor Cowardice, and a chance at afflicting the Diseased status effect which deals more damage while applying another healing debuff in Minor Defile.
Nightblade has numerous ways to guarantee the hit. 2 mobility skills (one is always active on either bar), a gap closer, an AoE instant fear, a single target instant stun, a teleport, and a single target immobilize/snare. It might suck for your build, but this is what it was designed around. It's a very strong skill with a lot of effects that a full or 2/3 Nightblade would have no trouble meeting the conditions of.
You ask if you really have to give up on Nightblade, but you yourself admitted that Siphoning was the most unnecessary part of your build and are you really even a Nightblade at that point? You may be attached to that specific character, but you're no more a Nightblade than a Warden or Templar currently.
If you are attached to the character instead of performance I'd suggest dropping it for Degeneration or adding Mist Form to your bars. A mobility skill you can use to teleport into the enemy and directly cast Power Extraction. Vampire and Siphoning also fit together very well thematically.
You'd end up getting the 300 weapon/spell damage from Vampire's Strike From the Shadows passive + Major Brut/Sorc + Minor Courage + debuffing them with Minor Cowardice and potentially hitting them with Diseased.