It's a matter of personal preference.
For the last couple of years,
MVMCP ticket holders have been able to get into the park with just their party ticket after 4 pm. When the park closes to day guests (usually at 6 pm), the ticket holders are asked to go to one of the "Lands" to wait. IIRC, this was Tomorrowland last year. Rides and at least one counter service restaurant remain open to party guests, who should have received a wristband when they entered.
We have eaten at the LTT and CRT while waiting for the CMs to get the park ready for the party. It's a great way to spend that time and when you are through eating, you are already in the park - no, they don't make you leave, but if it is not yet party time (7 pm), the CMs do ask you togo to Tomorrowland for the rest of the time.
One year, we just entered at 4 and grabbed a quick bite at the counter service restuarant in Tomorrowland. That was nice, too.
So do whatever you think best/prefer to do for dinner. If you eat outside the park, though, you may not be able to enter between 6 and 7. Not sure if they continue to let MVMCP guests in after the MK closes to day guests - it's easier for the CMs to get the park ready if there aren't lots of people around. They might make you wait until 7 if you arrive after 6.
Best wishes -