Like many other posters here, my Harmony remote stopped properly selecting the Cast input when my P75 was upgraded to 18.104.22.168. While I've seen the posts on how to fix this by telling the Harmony app to select the Component input instead, I believe I've found a better approach.
What I did was delete the Vizio P75-C1 from my Harmony and add a Vizio M65-C1 instead. I chose that model as it was one of the more recent models before
Vizio shifted from their own Smart TV platform to the current system of embedding Google Cast. I figured this would give me access to a wider array of Vizio TV IR commands and that one of those would give me direct access to the Cast input via a discrete IR code. I had previously tried this approach under the old firmware but had no luck. I'm not sure if I just missed it last time or if this is a feature of the new firmware but the M65-C1 command set does indeed now have a discrete IR code that selects my P75's Cast Input.
The command in question is called VIA in Harmony's database and the input Harmony has it associated with is called Vizio Internet Apps. Once I figured this out, I went and edited the list of inputs on my "M65-C1" device and deselected all the extraneous inputs (such as the ones they had for Amazon and Vudu) until I was left with inputs our P-Series TVs support: HDMI 1-5, Component, and Vizio Internet Apps (aka, the Cast input). I then reran setup for all my TV related activities and input selection is now working flawlessly and without the annoying Input Component, Input Next, Input Next sequence the Harmony does when configured to use the P-Series command set.
While I was at it I tested all of the other M65-C1 IR commands to see if they did anything useful. Most didn't but I did find a few that were useful:
FastForward & Rewind: Act like a "next/previous chapter" buttons in some Cast applications such as Netflix and YouTube. So, if I'm binge watching a series on Netflix (or have queued up several videos on YouTube) I can jump to the next episode/video by pressing FastForward. This is a nice way to blast past the 30 second count down Netflix shows before starting the next episode.
Stop: Exits the current cast application and returns to the Cast's slideshow/backdrop.
One additional discovery I made along the way was that the "Play" command Harmony had under both the P75 and M65 profiles was causing the next "Pause" command to be ignored. In other words, if I would Pause a Cast video, then resume with Play, the next time I wanted to pause, I'd have to press "Pause" twice. I don't recall seeing this discussed before but I've only been following this thread for the last few months so maybe I missed it or maybe this issue is unique to my particular setup. Anyway, my solution was to map my Harmony's Play and Pause buttons to both issue the "Pause" command. This seems to work much better for me and I no longer have to remember to avoid the Play button when Casting.
Finally, the M65 command set doesn't have the "Link" command that's needed to pair the TV with the Tablet. I simply used the Harmony learning functionality to add that command using the mini remote that comes with the TV.
I've been using this setup all weekend now and feel that my Harmony remote and Vizio are working perfectly together. It is also nice to have the additional transport controls even if they are still pretty limited.