yes i think many noticed this .even Vansh remembers the dance he did with Riddhima in flashback in the next morning . There are times when he does stare her intensely and it didnt seem to have anything with his suspicion I think they might reveal it later on why he married her bcz he could have simply forced her to live in the house on pretext that he doesnt accept her resignation .
His dialogues ye mangalsutra tumhe mujse kabhi dur nahi jane dega and the sindoor and bangle one wasnt needed at all . He has hidden motive behind marrying her apart from knowing her truth .
Vansh is definitely fascinated by her. He keeps on forgiving her mistakes each time. He keeps quiet when she shouts at him.
He gives a great deal of respect to his marriage even when it didn't happen under normal circumstances. All of it shows there has to be another motive other than just knowing the truth.