Correct -- you get your free 30 days, and if you enter a Credit Card / gametime card during registration the additional time will be used only after your free time has expired.

The 30 days also "stacks" with the free trial (if you choose to use recruit-a-friend or otherwise) such that you will get the free trial time + 30 days with the purchase of vanilla WoW, and only after that is used up will you use the gamecard/credit card time.