Hi Fiona,
I'm glad I was able to help you with your previous question :-)
Generally meals at Walt Disney World take over an hour, which would not leave you much time to get to Fantasmic, but dinner at Hollywood & Vine is a buffet so you can dictate how long you spend in the restaurant. A great time saver, as there is a set price, is to ask your server for the bill when you are seated, explain that you want to make a swift exit after you've eaten and they should be happy to help.
Fantasmic is being shown at 8pm on the day you visit and normally I would recommend you get in line to secure a seat between 60 and 90 minutes prior to show time. However you're visiting at a quiet time of year so you should still be able to get a seat 30 - 45 minutes prior, this gives you about 45 minutes to eat. Make sure you have studied a map prior to your meal so you know the quickest route to get to the amphitheatre.
There are Fantasmic Dinner Packages available at the H&V which gives you a voucher for entrance to preferred seating at Fantasmic. There are only a limited amount of these packages available and they need to be booked in advance by calling 407-939-3463. You can see some details on the package
HERE.
Have a great trip and I hope you manage to see Fantasmic,
Jo