I am trying to determine the cheapest way to learn an IR command from a remote. Whether it is some sort of USB device on the PC and running some software, or whether it is just a cheap (the cheapest!) universal remote that has learning capabilities. Please recommend!

For some background info ... I have a Harmony 700, which for the most part works perfectly. The one issue I have is that the command set for my Vizio TV (M470SL) does not contain discreet IR commands for input selections. However, the command set for the Vizio E420VA DOES contain discreet commands for input selection, and those commands actually work perfectly with my Vizio M470SL.

I still want to be able to use the "real" command set for my M470SL, but want to add the few discreet commands that I need from the E420VA command set. The only way I can see to do this is to use the E420VA command set, teach the commands that I need to a temporary device (cheap remote, or PC, or whatever), add my M470SL back to my device list, and use that temporary device to teach the commands back to my Harmony 700 for the M470SL.

Among other issues, I am maxed out on the number of devices that my Harmony 700 supports, so I can only have one TV in the device list at any time.

Any thoughts or ideas would be appreciated.

Cheapest way to get these discretes into your Harmony is to ask Logitech to add them.  Send them the Pronto Hex codes in this thread for the discrete codes in question.  If they aren't willing to do that, see post #21 of the linked thread.  Buy a Comcast remote from eBay, and follow the instructions in post 21 to make the Comcast send the desired IR signals.  You can teach those to the Harmony. Some other cable company remotes will also work.

Thanks, 3FG. I did, after your reply, go ahead and have Logitech support add the codes for me to my remote. They did that very quickly (within a couple of hours) and the codes work perfectly.

It's less of a 'crisis' now that things are working, but I think I'll still poke around for a very cheap universal remote that has learning capabilities. I like the idea of having the flexibility for adding codes to the Harmony remote myself, without having to open a support case every time and wait for them to do it.

I saw that Bestbuy has a learning remote for $20, which seems reasonable. I'll also keep an eye on eBay, as per your suggestion. Unfortunately, my cable box remote doesn't have any learning capabilities.

Thanks for the ideas.

The point is that you don't need learning capability, because if you do have a source to learn from, you could  teach the Harmony directly.  Instead you need a way to input IR signals for which you don't have an actual IR signal to learn from-- for example discrete IR signals not present in the OEM remote.  
The remotes made by UEI (the Comcast is one example) can be manually programmed to send additional functions besides the ones in the OEM remote.

I'm not sure where you're based but a cheap learner would be the Philips SRP5004 (Europe) or SRP5107 (USA)