Hey there Megan,
I can just imagine how excited your are about your upcoming Walt Disney World vacation. Visiting Walt Disney World during the holiday season is such an amazing experience. You're going to make memories to last you a lifetime!
While there haven't been reports of a Meet and Greet for Wreck-It-Ralph and Vanellope von Schweetz during Mickey's Very Merry Christmas Party, you can meet with them at
The Magic of Disney Animation in Disney's Hollywood Studios. My 4 year old princess is obsessed with Vanellope, so we always make a point of visiting her during each of our vacations.
Character Meet and Greets are subject to change, so be sure to check the Times Guide for Hollywood Studios to find out when Vanellope and Ralph are available for a visit.
I hope this answer helps you with your vacation planning, Megan. If you have any other questions, please feel free to ask at any time. We wish you a wonderful holiday vacation, and hope to see ya real soon!