Yeap Ive tried that
http://mmsc.vmobl.com in so many combos its not funny. What I have found in hours of testing:
MMS will not work without the cinet line. At least never has for me.
If "mms" is included with the n.vmu.ispsn line it would work (MMS wont LTE will). I would agree its a port/proxy/address/username/password issue.
Needle....meet haystack.
Ive tried to run logcats as well - didnt really give any usable info. When running the Force CM ROM what APN lights up? (Settings > More... > Mobile networks > Access Point Names)
After hours of Google searching I have found no one really knows the Virgin US settings. Its not posted anywhere from what I can tell - just shots in the dark like mine.