EDIT - found this great work around on the TMO forum.
"07-24-2010 02:09 PM
How to assign a ringtone to gmail contact on Samsung Vibrant This question is Assumed Answered. These steps are used for how to assign a specific ringtone to a gmail contact on the Samsung Vibrant. You will not be able to do this if the contact is on the sim card, facebook, or gmail as of now. (Future updates may change this).
1) On home screen tap contacts
2) While in the phonebook tap menu, then more, and click on import/export
3) Then select export to sd card ( it will then ask you to confirm your selection, select yes. This can take some time based on how many contacts are on the device).
4) Once export is completed, access the phonebook again and tap menu, then more. Go back to import/export
5) Then select import from sd card, a screen will then pop up and give you the option as to where you want to save the contacts.
6) Options: Gmail or phone or if you have a corporate email this will also be an option. Make sure you import them to the phone in order to assign a ringtone to the contact.
7) The device will then start reading from the vcard on the device. (This will also take some time based on how many contacts are on the device). Note: Now each contact will then have the little g for Google and phone icon next to the contact in your phonebook instead of having to create seperate contacts.
You can now select a ringtone to an individual contact two ways:
1) From the music player long press the ringtone you want to assign and select "set as", then select where you want the ringtone assigned. You will have 3 choices: Voice call ringtone, Individual ringtone, and Alarm tone.
2) On home screen tap contacts (do this after you imported the contacts from the sd card), then long press the contact you want to have the ringtone assigned, then select edit, then scroll down and select the desired ringtone and select save. Note: With contacts that are saved to the phone, this will also allow you to assign individual caller id photos."