FYI, I have hacked up a quick&dirty Plasma Mobile quick settings drawer button for VPNs:
You do not need to compile anything for it, you can just drop the contents of the linked folder into a new folder called ~/.local/share/plasma/quicksettings/org.kde.plasma.quicksetting.vpn (where ~ is your home directory) on the device (PinePhone, PinePhone Pro, or any other Plasma Mobile device).
The set up still requires the desktop kcm_networkmanagement, so you need to scroll around a bit in that dialog to configure the VPN. If you have no VPN set up yet, the quick settings button just brings up the dialog for you. Otherwise, it takes the first VPN it finds (there is no selection yet for the case where there is more than one) and toggles it, i.e., disables it if it was enabled, enables it otherwise. Depending on the VPN type, you will most likely be prompted for secrets with yet another desktop dialog, but it allows you to connect.
I doubt the code can be included upstream as is because the code architecture is just ugly (scraping a string to check the status and invoking a shell script to do the toggling using nmcli), but if you want a convenient way to use a VPN under Plasma Mobile as long as upstream does not support it officially, there you go.