Honestly, even after all this, I cannot bring myself to dislike Vinu at all. It is easier for us to see that Pakhi is wrong, but for Vinu, siding with Pakhi, his mother, is what comes most naturally to him.
Also, Vinu's hectic behaviour is also something understandable. He was always shown to be a sensitive kid, and think about all that has happened in his life recently. He had this image of a family in his mind, and suddenly that entire image has been broken. Everything he has ever believed in is suddenly a lie. That kind of knowledge and that kind of change must have been too emotionally stressful for him.
At this point, he doesn't know who his real family is, so, he is siding with the person who is showing him the most attention, and proving herself to be the most honest and reliable. Unfortunately, for him, the one person who he believes to be his true family is the one who is betraying him the most.
Vinu doesn't believe in the jaadugarni story in the sense that it is real. But his subconscious mind has understood it in the way that Sai is the bad guy who is breaking his family. And why shouldn't he think that? As far as his almost a decade-old life is concerned, he has no reason not to love Pakhi and Virat, and now there is suddenly a woman whose presence single-handedly is making him question his reality.
Moreover, he is confused, for Pakhi, the person whom he trusts the most in the world, is telling him that Sai is wrong, that she is the villain, but in his heart, he has seen her love and care. And he knows deep down that Sai is not the person Pakhi is making her to be. He understands that Pakhi's entire jaadugarni story is not fully fitting the situation.
But still, whom can a 9-year-old believe? The woman who has just entered his life, or the one who has been his mother ever since he was born? So, in that state, he is being with Pakhi, but there is a frustration inside him because he knows that something is wrong, but he doesn't understand what, and since it all began after Sai came back, he is channeling all his frustration, all his fear, all his anger, against Sai.
Basically, his seeing Sai's love and care is not what makes his action weird, but it is what is fuelling it.
His state is not just pitiful. It is heartbreaking.