How can I turn off JUST the notification sound/vibration when emails come in. I like the visual notification, but the sound/vibration is overkill for me.
You can do that (on modern Android versions) using the system notification settings, even on a per-account basis. Press the bottom item (“Notification settings”) on K-9’s notification settings screen (notice that you can click the text, not only the checkboxes)
Quite often, actually That system notification settings screen seems to be unintuitive for quite many people. Maybe Google will change it in a future Android version.
That was a good hint. However on my system (Xiamo Mi 11, Android 13, K9 v6.804) I did not find a way to access those setting from K9 and had to go through the OS notification setting app and choose K9 from there. The menu itself is perfect, but too bad it is hidden from the K9 app. That should certainly be improved in the app.
In K-9 go to Settings, the account you want to edit, then Notifications. There you can either tap directly on “New Mail Ringtone” to change it for that account, or a little farther down tap on Messages. This will open the native Android notification settings where you can select a sound (or None), enable vibration etc.