While I agree with you that Vikrant's role comes more natural to him, I have to bring up the fact that Asad's character is very distinct. Very angry young man type. Not many actors can relate to this type of character in real life, especially not Karan who has himself admitted that the character is very different than his real life personality: Bubbly and joyful.
That being said, it is easier for Vikrant to fill a character than for Karan. Thus, I would have to respectfully disagree with you and say that Karan has much more of an acting challenge than Vikrant. What is more that he has completely filled that expectation with finesse. For one, his subtle acting. He moves his head a milimeter and we know what's going down.
His dialog delivery style, etc. has truly morphed his character. He truly embodies Asad during times of shooting. From the eye rolls, smirks, passionate eyes, his acting style is very subtle - something much more difficult to do as an actor. And because the character of Asad requires this subtlety Karan fills it by not overacting or being dramatic. But simply becoming the character effortlessly.
Lets not forget his presence is the root of this show. Ayaan may be replaced, but Asad can never be.😳