If you are accepted on the first go around, Dr. Zic will be able to notify you after the 15th (or whatever the national date is this year). You will subsequently receive a written offer of admission.
A few people don't end up getting called, and I assume it is for accidental reasons since Dr. Zic really wants to be able to personally let each accepted student hear the good news from a phone call as opposed to a letter.
If you are not accepted on the first go around, you will be put on a waitlist. Vandy doesn't really reject people (at least those who got interviewed) until April/May, regardless if you have a good shot or not to get off the waitlist.
Don't worry too soon if Dr. Zic, doesn't call you on the 15th. I was called on the first day he was calling a couple years ago and it was several days after the 15th. Also keep in mind that he might not have time to call all 20 (or however many there are) accepted students in one night. He does is calling in the evening, just to let you know.