first: the fact that this is a VZ branded phone is irrelevant to the perfomance one will receive when using it in GSM mode. The baseband is capable of utilizing all GSM and CDMA frequencies currently in use (and I think there is an LTE RIL as well, but, I am high at the moment,)
The phone will get the full "FauxG" (3.75G) performance on all systems that use the 1900 mhz allocation that the ATT towers and the re-farmed T-Mobile towers do.
Why use a Verizon phone on say Tmobile? Simple economics: Because most people think these phones are locked to VZ on ebay (and I am sure everywhere else) these awesome smartphones that are VZ branded sell for a fraction of what they would if marketed as 'unlocked'.. (Which they are!! SHHHH!!!!)
The problem with the MMS is the APN, the disappearing / weird behavior of that and some related settings has to do with the 'build.props' settings file found in your device's /system/ or /system/bin/ folder... This file was written for VZ CDMA... A few very minor tweaks for us GSM users (like ro.ril.properties.APN-Persistence = true !!!)* and all those little hassles disappear like so much software sorcery... (that was 'joke syntax', dont use on your phone, where there would be no caps and the true would be indicated by the digit '1'.)
MY DNA arrives next week, will post the changes you should make for full GSM functionality without an active VZ data connection at that time. Or, post here (or im me) a copy of this file from your phone (need root and a file manager like root explorer.) (File is a few KB of human readable text) and I can make a list (quite short) of the needed changes (ro.android.properties, ro.ril.properties etc,...)
Jason Bender
Previous Gigs: Atrix Post-Release Dev-team