Louise,
There is NO reason for Android to disable it or to display a warning (unless the warning is to avoid trusting Verizon too much). The Verizon rep you emailed with was either ignorant or lying outright. :angry:
You CAN use ringtones bought online and they definitely do NOT need to be from VCAST. I have over a dozen non-standard ringtones I did NOT buy from Verizon. You just need to get them from a reliable source (be CAREFUL, not everything that shows up in a Google search is safe) and make sure they're standard MP3 files.
In addition to buying ringtones online, you can get free versions of most ringtones if you look carefully in the marketplace for an appropriate ringtone collection.
You can also make your own free if you already own the music. Just rip the song from the CD to an MP3 file, tweak it using a freeware audio-file editor (Audacity from audacity.sourceforge.net is a good one if you're using Windows), copy it to your SD card, and select it as a ringtone or alarmtone.
Finally, if you're using an Android phone and a Verizon rep tells you that you need to pay them more to do something as basic as this, think twice and then do some research... They're usually wrong.
Good luck.