Virgin more than likely will not work out to seas (they are a subcompany of Sprint, so use the Sprint towers.) Virgin and Sprint use CDMA technology, meaning they do not have sim cards= no international use (or extremely limited; Sprint phones might be able to roam, but they usually do not allow their prepaid to). My suggestion would be to pick up a cheap ATT go phone (they run anywhere from $14-100) so that way you can use your phone on the cinuglar/ ATT service, and won't have to worry about going over your minutes and seeing crazy fees.
Edit: I've been working with Virgin for a few years now, and unfortunately their customer service will almost always tell you anything you want to hear
I'm 99% positive the Virgin phones will not work internationally