title: How to implement vault functionality in braintree to show existing card info to user
body: <p>I have integrated Braintree .net core API I am able to create a token using generate token method.
  first time while passing client Id it is throwing an error.
  Also, I have Request new customer during the TransactionRequest using CustomerRequest method where I passed userId, I also added StoreInVaultOnSuccess to true.
  During the first transaction, I am able to create user but later it throws an error.
  So could you please explain how to use vault  functionality in .net core so that I can 
  displayed saved cards to the user.</p>