So, my bad, I was kind of excited when I made the first post about this, didn't mention every detail, was just happy to have vowifi and volte working properly!
To get this to work, a few things are necessary. You of course need Korean Open running on your device. It needs to be rooted and vbmeta disabled. You also need a root file explorer. Additionally, you need the OPEN_US folder from a device running US Open Pie (I'll provide gdrive share link).
1) With the root file explorer, copy the provided OPEN_US folder into directory ROOT/OEM/OP (you'll also see in this directory a folder called OPEN_KR).
2) Also in this directory you'll find a file called cust_path_mapping.cfg, edit this file to point to OPEN_US (instead of OPEN_KR)
3) Rename the OPEN_KR folder to something like OPEN_KR_ori (if you decide you'd prefer to delete it, be certain to back it up first).
reboot
That's pretty much it. Vowifi and Volte will work as expected, but, be aware, for both of those to work, your carrier and sim must be capable of them. Doing the above only makes the Device capable of them, if your carrier and sim aren't provisioned for them, the above changes will make no difference at all. And don't forget, some carriers restrict these abilities to only **their** devices.
This is the link for OPEN_US folder. It's from Pie, but it still works. About 75 meg in size (zipped).
cheers