SUMMARY
VPN passwords are not saved (when enabling "Store password for this user only (encypted)"), however WIFI passwords are. Not sure when this stopped working, but earlier versions did function correctly, so I suspect a regression.

STEPS TO REPRODUCE
1. Create VPN connection
2. Set password and enable "Store password for this user only (encrypted)"
3. Apply changes

OBSERVED RESULT
Upon navigating away from VPN connection dialog and navigating back, password is empty. Attempting to connect to VPN prompts for password every time.

No map entry is created in kdewallet for the VPN, however WIFI passwords stored via the same method are successfully created in kdewallet.

Storing unencrypted works as expected, but is clearly not preferred.

EXPECTED RESULT
Password is saved and retrieved from kdewallet successfully.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch
(available in About System)
KDE Plasma Version: 5.16.4
KDE Frameworks Version: 5.61.0
Qt Version: 5.13.0

ADDITIONAL INFORMATION
Saving the VPN produces the following logs, that seem to indicate the correct calls are being made, however no entry appears in kdewallet:

audit[609]: USYS_CONFIG pid=609 uid=0 auid=4294967295 ses=4294967295 msg='op=connection-update uuid=f8f805f3-5bb3-4fdd-9d66-f4890ec081f4 name="vpn_connection" args=vpn.secrets pid=17990 uid=1000 result=success exe="/usr/bin/NetworkManager" hostname=? addr=? terminal=? res=success'
NetworkManager[609]: <info>  [1567404184.1217] audit: op="connection-update" uuid="f8f805f3-5bb3-4fdd-9d66-f4890ec081f4" name="vpn_connection" args="vpn.secrets" pid=17990 uid=1000 result="success"
kdeinit5[12397]: plasma-nm: virtual void SecretAgent::SaveSecrets(const NMVariantMapMap&, const QDBusObjectPath&)
kdeinit5[12397]: plasma-nm: Path: "/org/freedesktop/NetworkManager/Settings/10"
kernel: audit: type=1111 audit(1567404184.118:253): pid=609 uid=0 auid=4294967295 ses=4294967295 msg='op=connection-update uuid=f8f805f3-5bb3-4fdd-9d66-f4890ec081f4 name="vpn_connection" args=vpn.secrets pid=17990 uid=1000 result=success exe="/usr/bin/NetworkManager" hostname=? addr=? terminal=? res=success'

Opening the connection edit dialog produces the following logs, however there's nothing in kdewallet, so passwords are predictably empty:

kdeinit5[12397]: plasma-nm: virtual NMVariantMapMap SecretAgent::GetSecrets(const NMVariantMapMap&, const QDBusObjectPath&, const QString&, const QStringList&, uint)
kdeinit5[12397]: plasma-nm: Path: "/org/freedesktop/NetworkManager/Settings/10"
kdeinit5[12397]: plasma-nm: Setting name: "vpn"
kdeinit5[12397]: plasma-nm: Hints: ()
kdeinit5[12397]: plasma-nm: Flags: 4

Tested using both OpenVPN and L2TP VPN connections, both produce the same result. There are other bugs filed against specific VPNs, however this appears to be a general problem, rather than specific to a certain VPN backend.