You don't need to carrier unlock for mods, you just need to unlock the bootloader and flash a custom recovery. And there's a free way to carrier unlock the phone, but it requires S-OFF. I can help you with the link if you're interested. For me, personally, there's no reason to S-OFF unless I want to make flashing Custom ROMs easier by allowing the zip to flash the kernel and antennas itself. But no, unless that SIM had an unlock script for the Vivid (which I doubt from Wal-Mart), your phone (and more than likely the SIM as well) is still locked. Now, if you want to mod your phone, I'm more than happy to oblige. Just private message me and we'll talk more.
Sent from my HTC Vivid WCX using Android Central Forums