Ruthless Stroke is very nice and indirectly offers fusion as well (2 steps) but doesn't synergize with SA/TA as well as Rudra/Mandalic, I believe.
You are mistaken here. Even before PDL on Mpu Gandring, main hit of Ruthless is still stronger than main hit on Rudra. Ruthless is the strongest SA and TA WS.
Good to know, I thought that was the case only when you could benefit from PDL.
Also to be more precise, before PDL Ruthless main hit is the best SA or TA WS among dagger WSs. Savage with Naegling is overall the strongest. SA or TA Rudra with Vajra is also stronger than with Twshtar if I remember correctly, and SA+TA Rudra with Vajra might be overall even stronger than SATA Savage.
there's also a strong argument that a THF is never safer gear-wise than in a proper Vajra AM3 set over other options. I know its not what everyone wants to talk about vs DEM DEEPS, but its a reasonable addition to the discussion.
I love my Vajra, and while its not king of the kill, its miles away from bad or worthless.