There is evidence that Sodium Benzoate reacts with Vitamin C to create Benzene, but that is a reaction for those two pure compounds. There are low levels of Sodium Benzoate and no Vitamin C in the formula that would cause a reaction.
There is no current evidence to show that this reaction would happen within the body if a person was taking Vitamin C supplements. If using the toothpaste as directed the Sodium Benzoate would be extremely diluted even if swallowed.