Among the store contact employees, how many of them have a Vista credit card?
store contact refers to PersonType = 'SC'; type of credit card refers to CardType; CardType = 'vista';
SELECT COUNT(T1.FirstName) FROM Person AS T1 INNER JOIN PersonCreditCard AS T2 ON T1.BusinessEntityID = T2.BusinessEntityID INNER JOIN CreditCard AS T3 ON T2.CreditCardID = T3.CreditCardID WHERE T3.CardType = 'Vista' AND T1.PersonType = 'SC'