> I've tried to set Vref- and Vref+ to 1.5 and 2.5V respectively,

This is out of spec.

> I've found no reference in the documentation to what the Vref's could
> be, and what it does to accuracy (I only need 8 bits).

See "PIC 18F87x Data Sheet" (DS3029C), table 15-12 on page 174, parameters
A20 - A22.


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