ok this is what i have:

under connections-> edit Tmo-mms
connection type: gprs
access point name: wap.voicestream.com
then under advanced
use proxy: checked
http proxy 216.155.165.50
port 8080
and everything else blank

now inside fmms
i hace my phone number xxx-xxx-xxxx
picture size medium
connection mode polite

and under configure
access point name wap.voicestream.com
proxy 216.155.165.50
port 8080
mmsc: http://216.155.174.85/servlets/mms
and everything else blank,
all settings but the mmsc under configure get populated from you VPN, i think

all that works for me