help recording with a Yamaha P-125 digital piano

Most welcome, doc3333! Again, would appreciate a screenshot of Logic's window. It's hard to understand what's going on in your project just from verbal description alone. It could be a number of things, and without a picture all that is left is guessing. It looks like the terminology you use is confusing. For example, it's not clear how you're recording your keyboard part. Is it as External MIDI or is it as a Software Instrument?

If it's the former, you will have to connect audio output(s) of your keyboard to one or two of the Scarlett's inputs set to LINE. Create an audio track, and select the Scarlett's input(s) where Yamaha is connected. Now, I don't know the exact capabilities of Yamaha. It might as well be able to transfer both MIDI and audio via its MIDI cable. If it can do both, that may potentially change the recording setup.

If you're recording the piano part as a software instrument (using one of Logic's instrument plugins) you won't need to do all of the above, since it's the plugin that produces the sound. Make sure none of the tracks is muted or solo-ed in the project. And again, please do post a screenshot.

From what you say, you're able to record on MIDI-related tracks in Logic using your Yamaha piano via USB. The issue is with recording audio (vocals), correct? Or are you also trying to record Yamaha's audio output? If possible, please provide a more detailed description of your setup and what are you trying to achieve. A screenshot of Logic's window (along with a few comments on what's what here) might also help.

Recording audio with Logic isn't rocket science. What you need to do is create an audio track, choose a correct input on its channel, and record enable it. Your audio interface should also be set as input source in Preferences > Audio (General tab). I cannot tell why "the piano becomes inaudible" exactly, because it may be a number of things, depending on what you're trying to do with Logic and how it is set up.