The troubleshooting page that Christopher linked to is not very good, unfortunately. For instance, it doesn't mention the Wi-Fi Calling related problems at all, and those come up in the forum posts all the time.
To my understanding, your device should successfully do MMS messages even though they use data and you don't have a data plan on your account, because the messages are billed as texting and not as data. And as you said, it worked properly on your Galaxy Vibrant.
I'd recommend giving 611 a call to see if you can get this straightened out. It's probably just something on your account that they didn't set up properly for you.