I suspect the answer, in a nutshell, is LATENCY.

When you play the MP3 track the sound goes straight into the mix for the recording. But what you hear to play along with might have slight latency (delay) by the time you hear it in headphones or via speakers. Then your sound going back into vMix via the HD500X has some latency too - about the same latency as occurs in the video input, which is why you are are sync with the video from your camera, but both of those are delayed behind the MP3 track.

One way of fixing this is to play the MP3 from an external player (software or hardware) and take the audio into vMix via a similar interface to the HD500X so that both have the same input latency. Or you could create your audio mix externally (with a hardware mixer) so that what you feed into vMix is effectively the master audio mix.

It will if you can perform the mixing in the POD and feed that mix to vMix. Presumably you know how the POD works, since you own one (and I don't)?

I did have a look at the manual to try to find out about the MP3 input, and it says:-

17. CD/MP3 Input - Connect a CD player, MP3 player, drum machine
or other device here, and you’ll hear it through the Phones,
Unbalanced Output and Balanced Output. Use your
device’s output control to adjust the volume relative to your guitar
tone. Start with the volume on your device at its lowest setting and
bring it up to the volume you desire.

That implies that you would need to be using either of the "Phones, Unbalanced Output and Balanced Output" for your feed into vMix.

And did you use either the "Phones, Unbalanced Output and Balanced Output" for your feed into vMix, as I suggested? The bottom line is that vMix isn't designed for doing what you want to do, in the way that you were trying to do it (i.e. using vMix both as the MP3 player and mixer). Your problem can be solved, but not by using vMix as the player AND feeding that signal directly into the final mix. What goes into the mix needs to be delayed, and to do that means taking it out and back in again, one way or another.

Using a "Virtual Audio Cable" could work. (1) Send the MP3 audio ONLY to a bus (e.g. Bus A) and NOT to the Master. (2) Set the Bus A output to go to the virtual audio cable. (3) Add a new audio input and select the output of the Virtual Audio Cable as the source. (4) Do a test recording to check the sync between this input and the guitar/POD and add a delay to either one as appropriate (in the vMix audio settings for the input). Once calibrated, save the project so that you can reuse it whenever required.