Hi Matt,
Below is TPC 29 on page 10 of the datasheet. This shows the VPHS output response (the triangular plot that references the "phase out -V" y-axis on the left and the phase error, in degrees, which references the right y-axis. At 0 degrees and +/- 180 degrees, the error is off the plot; something more than +/- 10 degrees. Low phase error occurs between 30 degrees and roughly 155 to 160 degrees.
At the end of the day, if your signals are an unknown phase that can be anything between 0 and +/- 180 degrees, then the AD8302 might not be the best choice for your system, given the large phase error.
An ADC with a low sample rate would work fine to measure the VMAG and VPHS outputs. They are both DC-ish outputs, in that once they are settled they do not move until either the phase or magnitude changes. They only have about 30MHz of video bandwidth each.
Yes, I would recommend some sort of front end receiver before the AD8302.
