You'll need some other phone. Put the sim in the new phone, call customer care, and say that you would like to switch to that phone on your line. Then have them remove the iphone plan and put media opt-out so you don't get charged anything accidentally. Give them the IMEI of the new phone. When you're done just put the sim back into the iphone.
When you're back from vacation call again and say you want to switch to iphone. It will take you 10-15 min. for each call.
Works 100%. They check accounts randomly from time to time and if they notice you have an iphone without a data plan they will add it, but in this case, it's only a month, they won't figure it out that fast.