using vodafone contract iphone 5 in usa

If your phone is carrier locked to Vodafone, no. The only way it will work in the US, as a phone, is roaming under an International plan from Vodafone.

Think extremely expensive, as in "sticker shock".

Talk to Vodafone about a holiday flatrate for your plan.

Or get it unlocked from Vodafone.

If you're in the UK, look at Dataroam for PAYG services