I have a similar problem.  Phone receives any and all messages, but MMS messages are stuck "sending" indefinitely. This was my work-around (but not a solution):

Turn Wi-Fi to OFF, then reconnect to VSW.  Create and send a new MMS message and it will go through fine.  (The original "stuck" message may also go through, but not necessarily). 

Other details:  Stock Samsung Galaxy Nexus, no 3rd party messaging app, data turned on and functioning, regular text messages working fine.