Assuming you are seated on time, you can probably expect the dinner to be at least 1 hour. Of the 3 times we've had dinner there, I've never been there for more than 1 1/2 hours. It all depends on when Cinderella and the step sisters get to you.
You can enter the VMCP at 4 so you will be missing at least 1 hour when you can possibly do things, but the park may still be fairly crowded at 5:30 as the people with general admission will trying to get in what they can before they have to leave.
The good thing with 1900 is that you are just one monorail stop away from the MK. You won't miss any of the festivities or holiday events either.