I think you can read the datasheet as far as ADCON1 goes.

Most PICs let you choose between two sources of Vref. You can choose AN3 or Vdd. If you choose AN3, then you must connect a reference voltage to that pin. All A/D readings will be compared against that reference.
If the reference is 3.0V the a 10 bit A/D will read 3.0V on any other A/D input as 1023. The "count" (1023) is equal to (input/reference) * ((2^10) - 1).
If you apply 2.0V to an A/D input, and the reference is 3.0V, the count will be 2/3 * 1023 = 682.
Everything is compared AGAINST the reference.

If you set Vdd as your reference, then every A/D conversion will be compared to Vdd.
Since your external chip puts out a constant 2.8V, if Vdd (the reference) is 5.0V then the A/D will output 2.8/5 * 1023 = 573
If Vdd drops to 4.0V, then the A/D will output 2.8/4 * 1023 = 716
If Vdd drops even further, the A/D output will go higher than 716


If you set