have the same problem and could not fix it, yet.
Please note: I'm talking about kubuntu 19.10 using vpnc. The solution which I use works fine with kubuntu 18.04.
So hints as in the link above do not help because in 19.10 (as int 18.04) a drop down menu is used where I try to use "Store password only for this use (encrypted)" (please note: I have a German install, so this is my translation of what I see in German) which (as already mentioned) works in 18.04 but does not do its job in 19.04.
I can use "Store password for all users (not encrypted)" which stores the password in 19.10 but as the label says the secret information is stored unencrypted in the conf file which is not what one would want to have.
So it would be more than helpful, if this bug would be resolved.
store for all users
work.