Hi Samantha!
Unfortunately for us early planners, the dates for Mickey's Very Merry Christmas Party have not yet been announced. However, according to the Disney website
here, the tickets will be going on sale in late Spring.
While things can change from year to year, I have noticed in previous years that MVMCP usually ends a few days before Christmas and so your vacation may fall just after the last party. Like I said, that might be different this year, so be sure to check once the dates are announced.
If in fact you do miss MVMCP, I have also noticed that similar parades and shows take place in the parks during the days of Christmas. So while you may not have the exact experience, such as the hot chocolate and cookies that are usually given out, or the feeling of being at a party with limited guests sold admission, you may find comparable festivities during your vacation. Again, these are all things that might change and have not been announced as of yet, it's simply the trend I've noticed in recent years.
I hope this helps, and come back to the Moms Panel if you have anymore questions!