Since you are getting above 140k you are on EVDO.
you should take the usb dongle and a laptop out for a ride... if they improve in the general location then you might be able to fix the issue with an antenna of some sort... check out this site for some antenna information: »evdotips.blogspot.com/
(the layout is a bit confusing but scroll down and on the right is the list of topics).
also don't switch off your $40 plan or you wont get it back...
and yes talking to VM customer service gains you nothing... I consider VM a self service type of product
Virgin Mobile forum is here: »Virgin Mobile Broadband
edit: Just got to thinking you could also just wander around your house/apt/whatever and see if you can get better speeds and if so re-position the cradle point at that location.