Hi Mark,
The force must be strong with you! The Florida heat would scare me away from a Darth Vader costume. Please remember that even though it will be October, the temperatures in Walt Disney World can be frighteningly warm! When you say you will be at the Magic Kingdom park for Halloween, I am hoping you mean you will be enjoying
Mickey's Not-So-Scary Halloween Party. From the "Know before you go" section of the web page for Mickey's Not-So-Scary Halloween Party:
"Guests of all ages may wear costumes only to the Mickey's Not-So-Scary Halloween party event; otherwise, costumes are only permitted at Magic Kingdom park for Guests ages 9 and under.
All costumes must adhere to any guidelines as determined solely by Disney.
Please choose costumes that are not obstructive, offensive or violent.
Guests' eyes must be visible at all times."
When I read the guidelines I can understand why you might be concerned about a Darth Vader mask. If the mask could be considered obstructive or block the view of your eyes, you might want to consider another costume. I think you might want to also consider whether small children might be frightened by Darth Vader. After all, the party is supposed to be "not-so-scary!"
Wishing you a magical, not-so-scary vacation,
Amy H.