Normally, you can't make Vref larger than VCC.
So, what you end up doing is scaling your inputs so that they go through only the 0V-VCC range.

You should add your own protection diodes, to VCC and ground at the micro, and have enough series resistance from the input so that current into these diodes will be minimal even with 12V on the input.

Plan for the worst in automotive applications, you won't be dissapointed.

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