Let's assume you get the Vz iPhone 4s unlocked for foreign carrier. Then you insert the foreign carrier sim card, switch from CDMA to GSM. That means the Vz account is temporarily disabled and no roaming or data charges will be incurred, is that right?
If the above is correct, then what I plan to do next trip overseas is to use skype and forward all calls to my Vz number to my skype number, and then have skype forward to my foreign carrier number.
Just got back from Europe with my new 4S. Beware of something totally lame: If you restore your Verizon iPhone 4S while in Europe you will then have a partially-functioning phone in terms of data service, messaging, and calls.
This really screwed me as I had to restore my iPhone for a software glitch and wound up with a device I could not do any texting, no data, and some calls. There is NO way to solve that problem until you return to the US and the phone is activated OTA with Verizon. Very lame issue I would have loved to know before I decided to perform a restore. This is apparently limited to the iPhone as full activation of the phone requires iTunes and for a Verizon customer to have the phone physically in the US to fully activate.
About using it in Europe, it works similar to the AT&T iPhone 4s; it will automatically find a GSM network. Just make sure to get Verizon to put a roaming data plan on your account ($30 for 50mb). One interesting annoying difference I noticed with the Verizon vs AT&T iPhone 4s was the fact that the Verizon phone does not allow you to select which European carrier to choose, like the AT&T model, it just chooses one for you. Annoying for sure.