If M is innocent, then M's statement is true. With "only one true," both B and V must be false.
If B is false, B is involved. If V is false, then "B is involved" is false – a contradiction (two truths).
Therefore, M cannot be innocent; M is involved (M's statement is false).
Either B is innocent (B true, V false) or B is involved (B false, V true), both satisfying "only one true."
⇒ The definite culprit is M.