Well..there are two options
I usually go for the logic one
Vansh AND vyom mentioned that the BB hold secrets of the underworld the possession of which would make its owner the king of mafia.
Even if there is something about the deaths of former mafia, why would that information make someone who owns it the mafia King.it doesn't make sense
While if Vyom had lied, knowing Dadi's fear about vansh getting to know the truth, dadi without knowing what the contents of the box is, may turn into an accomplice of Vyom which means a family member of Vansh themselves would get the BB to him.. It would be a smart move by Vyom and a more logical explanation.
But what i Don't understand is did Vyom take a photo of the tattoo??..he needed it badly right. Then why did he burn it without taking a pic of it..or did he?