I don't understand how it is that 1) you say that PXF gives results that don't seem correct and 2) you cannot get PXF to work. Those two statements seem inconsistent.
You can perform XF analysis of the LDO alone and PXF analysis of the VCO alone and multiply the transfer functions together, and that would give you an sanity check on the result from using PXF both of them together. The result will not be identical, but it should be very similar.
You would not apply a sine signal to the power supply when using PXF. PXF applies small-signal test signals automatically as needed.
I believe ADE provides a way of configuring PXF to allow the output to be phase. If not, you can apply the equations you find in
http://www.designers-guide.org/Analysis/rf-sim.pdf.
-Ken