So far the response from Microchip Support is that what I've done
should work if Vcc >= Vref+ and voltages are in spec, which it is
and they are

Leaving me feeling a bit insecure. Because I find it hard to believe
that no one before me has tried and failed to use Vref+ on this PIC

Then again, I thought the same about I2C until Microchip admitted to
a SSPIF silicon bug on the 16F88 running at 8MHz INTOSC, after
me pulling the chip and code apart for 2 weeks. And I note that that
bug is not yet (3 years on) in the F88 errata, even the specific SSP
errata sheet

Unfortunately in the 18F there isn't the wide choice in the low I/O range
as there is with the 16F, so I'll stick with the 18F1320 and modify the
circuit for 5.12V Vcc, unless MS can come up with something

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