That's wrong. Strong haki should certainly be able to block his attacks, at least, when used in the form of offense. Law's low end moves"multi building level" were being blocked by Smoker quite comfortably and casually. Vergo's haki was simply not strong enough.
I hope you understand that Vergo's haki is > Smokers. And Smoker managed to block his attacks, so its a given Vergo could aswell.
If you're talking about simply blocking his regular swings with Haki? That much is a given, Smoker can do it, Doflamingo can do it, plenty of characters with haki can do that.
I'm talking about his high end moves, I have no reason to believe Law's haki was > Vergo, heck I'm not even sure Law used Haki on Vergo when he cut him in half.
Zoro's haki is not gonna protect him from a mountain level slash. Nor is it protecting him from being sliced in half on regular swings without a proper defense (for example if Law gets past Zoro's swords, Zoro is getting sliced, he's not gonna sit there and tank it).
Where? I hope you won't be posting the scan of him blocking Doflamingo's strings.
Doflamingo attacks Law with sharp attack without hardening = Hardened
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with no damage
Law attacks Doflamingo with sharp attack without hardening = Hardened
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just from grabbing his blade.
I can't remember in this manga there being such a clear cut comparison to be made in terms of haki, both defenders were attacked with non hardened attacks, both defenders used hardening to defend, and both faced a sharp object. If this doesn't convince people that Doflamingo's armament haki was at the least not stronger than Law's then I don't know what does.