I did a factory reset on the Valvefx and plugged in the X15 via the MIDI In, so it's at least able to switch between programs on a 1:1 ratio. Now, the part I'm still stuck on is assigning the X15s Continuous Control to talk to the Valvefx.
I did the setup process on the X15 (holding mode and 1) and it showed that it was CC 4, which was what IntPedal on the ValveFx was assigned to. (The ValveFx manual doesn't do a good job of specifying the difference between Internal and External Pedals, but I think the Internal refers to the proprietary Ground Control pedal, which I do not have).
I then go to calibrate the CC pedal, pushing it forward is fine, but when it's time to push it back it reads "Calibrate error."
I suppose I could just try and track down the Ground Control, but I feel like the X15 can already do all of the same thing, it just simply won't play nice.