Hello!

The manual says that Yahoo Finance is the default for stock/mutual fund quotes:

https://docs.kde.org/trunk5/en/kmymoney/kmymoney/details.investments.prices.html#details.investments.onlinequotes

However, when I looked at the available options Yahoo was not listed:
https://pasteboard.co/Cxj9czTSAhQd.png

(I'm on Windows 11 & KMM 5.0.6, if that helps at all. )

Personally, I'm fine with not using Yahoo but I figured that I'd let y'all know - if it's not a bug then maybe the docs need to be updated?

Also, I can't get KMM to look up theVFORX mutual fund online no matter which online quote source I try, so it's entirely possible that I'm doing something goofy / wrong on my end.

There were certainly issues with Yahoo finance in the past, but Yahoo Finance is definitely working currently (at least for most users.)
5.0.6 is very old, so you should try to upgrade: see https://kmymoney.org/download.html for your options on Windows. There have been issues with the Windows builds, but I think most have been resolved. Definitely back up your data before trying a new version, and let us know if it works or not.
Regarding VFORX, I just tried it, and was successful in retrieving the price from Yahoo. What error are you getting? However, it may be better to wait until you get Yahoo working at all before digging into why a specific security isn't working for you.