There are limited Fantasmic dinner packages, so when you call you'll have to ask about H&V for the day you want to see the show. I think it's about $25 per person. They'll tell you what times they have open- you may have to eat at 4:30 p.m. After you eat, they'll give you little tickets that will allow you access to the special seating area and they'll tell you what time to head over to the theater. So you may have several hours between your meal and the show.
We did this at the H&V two years ago. The food was meh but we all found something we could eat. The desserts were a little better. Then, we got to the theater and just before the show was to start, thunderstorms started to roll in and it was cancelled. At this point, we were out of luck -you do NOT get vouchers for another show. We ended up just showing up another night really early and waiting for the show. It was spectacular but back then it was every night so it wasn't too bad getting in.