Hi, Laurel! Thanks for visiting me here on the Moms Panel...your question is a good one!
Just to make sure we're on the "same page,"
Mickey's Very Merry Christmas Party is a separately-priced and separately-ticketed event at the Magic Kingdom which is held on select nights (7 p.m. until midnight) during the holiday season. MVMCP guests enjoy special entertainment, parades, fireworks and treats not featured during regular park hours. Therefore, those MK guests without MVMCP admission have to make plans to exit the park prior to 7 p.m.
You may find that some MVMCP guests may arrive earlier than the party starts which makes for a more crowded "transition" time between regular park hours and party hours. Yet, for the most part, I haven't noticed that events (such as MVMCP, MNSSHP, grad nites or Night of Joy) make the park any more crowded during the day than normal. For the most part, what really matters most in terms of crowd levels is whether or not it's a weekend or weekday or whether or not the day you visit is close to a major holiday.
So...take a look at your overall daily itinerary paying particular attention to when parks close/open and if there are Extra Magic Hours available for resort guests. If your budget and your itinerary are flexible, consider MVMCP...it's certainly one of those events that make big impressions and memories for the family. Yet, if not, plan to spend the first portion of your day as you planned at the MK...then, consider other choices for entertainment that afternoon/evening such as another park, mini-golf, shopping or dining at Downtown Disney or an evening at Fort Wilderness or The Boardwalk.
Come back if you have more questions. Have a wonderful day and a magical vacation!