Guess some just don't know what durability is.
Bleeding is not the only way of taking damage. Sanji took Damage from Yuen, from Niji, from Drake, From Daifuku and from Judge (even though he was holding back).
If you want to compare durability between Sanji and his brothers, we have manga panels demonstrating just that.
One bullet proof , the other not
If there's no damage on the RS then it is the RS which is dudable. Has nothing to do with Sanji.
Body hardness is different from durability.
Vinsmokes would definitely take damage from those same attacks that Sanji was able to endure without bleeding, even if they were using RS.
The fact that Sanji's durability + strength at its base is already comparable to that of the vinsmokes whit exoskeleton + RS, is proof that Sanji is already a monster by nature and effort, and the durability of the suit is just an additional layer of protection.
It would be something like
Durability/Resistance of Base Sanji:
Note 6
Durability/Resistance of RS Ichiji:
Note 6.5
So the durability/resistance of Sanji with RS:
Note 10
No one from the germa would be able to withstand a lethal attack by the Zoan King, so that's a merit of Sanji