A, it is not only the carbon-carbon double bond that can discolor the acidic potassium permanganate solution, but also the phenolic hydroxyl group can discolor it. Even if ferulic acid is not produced, vanillin in the original reactant can discolor the acidic potassium permanganate solution. So, A is wrong;

B vanillin contains phenolic hydroxyl group, while ferulic acid contains phenolic hydroxyl group and carboxyl group, both of which can react with Na2CO3 and NaOH solution. Therefore, B is correct;

C, vanillin and ferulic acid can be added instead, but they cannot be eliminated, so c is wrong;

D, according to the conditions, its isomers are sum. When two hydroxyl groups and -CH2CHO are adjacent or alternate, there are two structures, including a methyl formate group and a phenolic hydroxyl group, and the positions are opposite, so one * * * has five isomers, so D is wrong;

Therefore, choose: B.