That being said I don't see the point. When connected to WiFi you should already be disconnected. You can check the status here. Setting / about phone / status / mobile network state should say disconnected.
Having recently traveled with a Droid Maxx, the global roaming setting definitely works. Verizon also has an international travel plan for reduced call and texting rates and 100 mb of data for $25 a month, which you can turn on for your trip and then turn off when you're home, in case you want phone and data for an emergency.
Well, it depends where you are, but as long as you're in a place that uses GSM (like europe), the phone has GSM radios, so it should work. Settings / More.... / wireless & networks / preferred network type. Make sure that it's global or LTE/GSM/UMTS.
Unless there's some sort of weird networking bug. In that case, you should still be good with global roaming turned off.