Amanda your in a tough spot because 1. people don't rsvp and 2. you kinda do need an idea since your getting villas. Are you going to see a lot of the important people during the holidays? If so I would just straight out tell them your situation about needing at least an informal headcount for the villas and then ask if they are going. It seems like a lot of people no they can't go but are afraid to say it, i'm not saying in yoru situation but it seems the case for a lot of it. So maybe stress to them, that you would completely understand if they can't go but you really would just like to know if they have decided. Goodluck!