Why are Haki attacks not effective on Marine admirals? Why are Haki attacks not effective on Marine admirals?

In the Marineford war, we see countless instances where Haki-imbued attacks don't affect Marine admirals. When Marco and Vista attack Sakazuki, he even acknowledges that they are Haki-imbued, but nothing apparently happens:



Another case is when Whitebeard attacks Aokiji: 



What's with the admirals? Their Logia powers don't seem to be easily affected by Haki attacks. I mean we have many instances where Haki has touched their body, but still there are many instances where attacks on their body don't result in damage. What's the deal with Marine admirals?
I think you are wrong in saying that they weren't affected. They clearly were affected as you can see from the wound on Sakazuki's shoulder. Since both questions have a different answer, I'll reply one by one.

Whitebeard vs. Kuzan



From Kuzan article:


  He then got stabbed by Whitebeard's bisento, to which pirates are seen wondering if Aokiji was stabbed with Haki and if he was dead. Aokiji told them not to spout nonsense before grabbing the bisento and using his Ice Block Partisan attack. All of a sudden, Jozu attacks him and Aokiji is caught off-guard, even bleeding from his mouth.


As you can see, Aokiji was not hit by the Partisan, which was most likely imbued by Haki. This is, because he opened up a hole, where the partisan would have hit, just before it would hit. If he hadn't he probably would have been stabbed to the heart. Marine admirals use their power smart and efficiently, but it doesn't mean they can't be harmed. This can be seen just a moment after, when Jozu attacked him with his diamond arm and he was seen bleeding from his mouth.

Marco and Vista vs. Sakazuki



From Busoshoku Haki article:


  Devil Fruit users who can use Haki can resist the effects of Haki-imbued attacks, Akainu was able to take Haki-imbued attacks while maintaining his intangibility by extending his Haki through his body in order to nullify his opponents bypass. This however depends on their level of mastery over Haki when compared to their opponent as can be seen when Akainu was in contrast harmed when attacked by Whitebeard's superior Haki and bled.


In chapter 784, it was confirmed that Haki can be bypassed. We can see how Doflamingo hits Luffy with a Haki-imbued attack, yet Luffy kept his elasticity during the whole event, bouncing Doflamingo's attack right back. The same must have happened with Sakazuki. It looks like Vista and Marco did hit Sakazuki, and he was affected, because he noticed the attack was different from that of non-Haki users. Though, despite being hit with a Haki-imbued attack, he wasn't harmed and could just reform his neck, because his Haki was superior. I guess Haki is a bit like armor. If the level of an attack is lower than the toughness of an armor, it will be unable to penetrate the armor, and unable to touch the body hiding within the armor.



The answer was updated due to wiki updates. Check the edit revision history for the old answer.