Riansh and the TRUST FACTOR
Riddhima's trust on Kabir is not something that she got in an instance, she was with him for 2 years and he manipulated her by presenting himself the most honest, loyal and lovable guy. Although there were some instances where he lost his patience and behaved as himself DSR and the chip fiasco.
We cannot call her dumb for that
Vansh's trust on Riddhima might got initiated after he saw her talking the bullet, caring for his family and looked over through her innocence. He had a doubt but still fell for her and trusted her. His trust too didn't come instantly, it was through a series of moments that made him trust her.
We cannot blame him for trusting his wife blindly just because he was introduced as a person who's totally calculative and doesn't trust even his shadow
Edit: mereko ab bhi aisa lagta hai vansh was planning something and was faking his trust on both Anupriya and Riddhima😆