I stick to some very archaic workflows, e.g. to connect to some corp VPN I just run sudo vpnc-connect and later on sudo vpnc-disconnect. In the past that also managed to restore my resolv.conf, currently it doesn't. According to a colleague that's also the case for Ubuntu.

Taking a step back, the sane way would be to use the NetworkManager vpnc plugin, but that does not work with this specific case because we use uncool VPN tech which requires the Enable weak authentication setting for vpnc. There is a feature request open for that one at https://gitlab.gnome.org/GNOME/NetworkManager-vpnc/-/issues/11

Taking another step back I thought that it shouldn't be that hard to add some checkbox, a boolean and render out another config flag or line in a config file. Not as intuitive as I thought this mix of XML and C. So let's quickly look elsewhere.

What happens is that the backup files in /var/run/vpnc/ are created by the vpnc-scripts script called vpnc-script, but not moved back, because it adds some pid as a suffix and the pid is not the final pid of the vpnc process. Basically it can not find the backup when it tries to restore it. So I decided to replace the pid guessing code with a suffix made up of the gateway IP and the tun interface name. No idea if that is stable in all circumstance (someone with a vpn name DNS RR?) or several connections to different gateways. But good enough for myself, so here is my patch:

vpnc-scripts [master]$ cat debian/patches/replace-pid-detection 
Index: vpnc-scripts/vpnc-script
===================================================================
--- vpnc-scripts.orig/vpnc-script
+++ vpnc-scripts/vpnc-script
@@ -91,21 +91,15 @@ OS="`uname -s`"

 HOOKS_DIR=/etc/vpnc

-# Use the PID of the controlling process (vpnc or OpenConnect) to
-# uniquely identify this VPN connection. Normally, the parent process
-# is a shell, and the grandparent's PID is the relevant one.
-# OpenConnect v9.0+ provides VPNPID, so we don't need to determine it.
-if [ -z "$VPNPID" ]; then
-    VPNPID=$PPID
-    PCMD=`ps -c -o cmd= -p $PPID`
-    case "$PCMD" in
-        *sh) VPNPID=`ps -o ppid= -p $PPID` ;;
-    esac
+# This whole script is called twice via vpnc-connect. On the first run
+# the variables are empty. Catch that and move on when they're there.
+if [ -n "$VPNGATEWAY" ]; then
+    BACKUPID="${VPNGATEWAY}_${TUNDEV}"
+    DEFAULT_ROUTE_FILE=/var/run/vpnc/defaultroute.${BACKUPID}
+    DEFAULT_ROUTE_FILE_IPV6=/var/run/vpnc/defaultroute_ipv6.${BACKUPID}
+    RESOLV_CONF_BACKUP=/var/run/vpnc/resolv.conf-backup.${BACKUPID}
 fi

-DEFAULT_ROUTE_FILE=/var/run/vpnc/defaultroute.${VPNPID}
-DEFAULT_ROUTE_FILE_IPV6=/var/run/vpnc/defaultroute_ipv6.${VPNPID}
-RESOLV_CONF_BACKUP=/var/run/vpnc/resolv.conf-backup.${VPNPID}
 SCRIPTNAME=`basename $0`

 # some systems, eg. Darwin & FreeBSD, prune /var/run on boot

Or rolled into a debian package at https://sven.stormbind.net/debian/vpnc-scripts/

The colleague decided to stick to NetworkManager, moved the vpnc binary aside and added a wrapper which invokes vpnc with --enable-weak-authentication. The beauty is, all of this will break on updates, so at some point someone has to understand GTK4 to fix the NetworkManager plugin for good.