As far as i am aware. as long as your phone is triband it can be used in the states. would be worth ringin up the helpdesk to find out if your phone is triband if you don't allready know.
Yes. Only bill pay customers can get roaming in the US. A colleague of mine recently went to the states and had to change to another provider thus allowing her to roam. It's a bit mean on Vodafone's part. They're losing out.
No Vodafone RTG roaming in the US I'm afraid. If the phone is unlocked and triband then you may be able to get a local SIM card - or just get a local phone if possible/necessary/appropriate.
There is little or no triband service outside the main urban centres in the US. They use a different system over there which doesn't require a sim card as all the details are built into the phone.
Better off buying a cheapo rtg local phone or renting one at the airport.
I have been over in the USA twice this year. If you have a triband phone and are billpay there will be no problem; otherwise you can't use it.
FYI, the only countries which you can roam in with Vodafone RTG are those which also have the Vodafone network which is most European countries and a few others e.g. Australia but definitely not the USA or Canada