I replaced the head unit with another of the same part number.

No code needed (2007 Vibe).

Neither your Vibe nor your sister's Vibe have an actual theftlock radio. Theftlock radios used by GM require a data bus in the car, where the radio is connected to the car's computer and other things. SInce the Vibe is mostly Toyota, there is no GM data bus in the car. GM uses the same basic radio ...

I have no idea if the OP is subscribed to this thread but if so, I'd like to know the resolution.

Thanks.