Comment 15 for bug 415076

Confirmed here (I believe it is the same bug) using vpnc and saved username/password.
An excerpt from /var/log/daemon.log:

Jul 15 18:59:22 JP-Portatil NetworkManager: <info> Starting VPN service 'org.freedesktop.NetworkManager.vpnc'...
Jul 15 18:59:22 JP-Portatil NetworkManager: <info> VPN service 'org.freedesktop.NetworkManager.vpnc' started (org.freedesktop.NetworkManager.vpnc), PID 13357
Jul 15 18:59:22 JP-Portatil NetworkManager: <info> VPN service 'org.freedesktop.NetworkManager.vpnc' just appeared, activating connections
Jul 15 18:59:22 JP-Portatil NetworkManager: nm-vpn-connection.c.828: NeedSecrets failed: dbus-glib-error-quark Rejected send message, 1 matched rules; type="method_call", sender=":1.2" (uid=0 pid=873 comm="NetworkManager) interface="org.freedesktop.NetworkManager.VPN.Plugin" member="NeedSecrets" error name="(unset)" requested_reply=0 destination="org.freedesktop.NetworkManager.vpnc" (uid=0 pid=13357 comm="/usr/lib/network-manager-vpnc/nm-vpnc-service))
Jul 15 18:59:22 JP-Portatil NetworkManager: <WARN> connection_state_changed(): Rejected send message, 1 matched rules; type="method_call", sender=":1.2" (uid=0 pid=873 comm="NetworkManager) interface="org.freedesktop.NetworkManager.VPN.Plugin" member="Disconnect" error name="(unset)" requested_reply=0 destination="org.freedesktop.NetworkManager.vpnc" (uid=0 pid=13357 comm="/usr/lib/network-manager-vpnc/nm-vpnc-service))

and /var/log/syslog:
ul 15 18:59:22 JP-Portatil NetworkManager: <info> Starting VPN service 'org.freedesktop.NetworkManager.vpnc'...
Jul 15 18:59:22 JP-Portatil NetworkManager: <info> VPN service 'org.freedesktop.NetworkManager.vpnc' started (org.freedesktop.NetworkManager.vpnc), PID 13357
Jul 15 18:59:22 JP-Portatil NetworkManager: <info> VPN service 'org.freedesktop.NetworkManager.vpnc' just appeared, activating connections
Jul 15 18:59:22 JP-Portatil NetworkManager: nm-vpn-connection.c.828: NeedSecrets failed: dbus-glib-error-quark Rejected send message, 1 matched rules; type="method_call", sender=":1.2" (uid=0 pid=873 comm="NetworkManager) interface="org.freedesktop.NetworkManager.VPN.Plugin" member="NeedSecrets" error name="(unset)" requested_reply=0 destination="org.freedesktop.NetworkManager.vpnc" (uid=0 pid=13357 comm="/usr/lib/network-manager-vpnc/nm-vpnc-service))
Jul 15 18:59:22 JP-Portatil NetworkManager: <WARN> connection_state_changed(): Rejected send message, 1 matched rules; type="method_call", sender=":1.2" (uid=0 pid=873 comm="NetworkManager) interface="org.freedesktop.NetworkManager.VPN.Plugin" member="Disconnect" error name="(unset)" requested_reply=0 destination="org.freedesktop.NetworkManager.vpnc" (uid=0 pid=13357 comm="/usr/lib/network-manager-vpnc/nm-vpnc-service))
Jul 15 18:59:22 JP-Portatil NetworkManager: <info> Policy set 'JP_Casa' (eth0) as default for routing and DNS.
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> (eth0): device state change: 8 -> 3 (reason 38)
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> (eth0): deactivating device (reason: 38).
Jul 15 18:59:23 JP-Portatil NetworkManager: <WARN> check_one_route(): (eth0) error -34 returned from rtnl_route_del(): Sucess#012
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) starting connection 'Auto eth0'
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> (eth0): device state change: 3 -> 4 (reason 0)
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 1 of 5 (Device Prepare) scheduled...
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 1 of 5 (Device Prepare) started...
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 2 of 5 (Device Configure) scheduled...
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 1 of 5 (Device Prepare) complete.
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 2 of 5 (Device Configure) starting...
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> (eth0): device state change: 4 -> 5 (reason 0)
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 2 of 5 (Device Configure) successful.
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 3 of 5 (IP Configure Start) scheduled.
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 2 of 5 (Device Configure) complete.
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Stage 3 of 5 (IP Configure Start) started...
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> (eth0): device state change: 5 -> 7 (reason 0)
Jul 15 18:59:23 JP-Portatil NetworkManager: <info> Activation (eth0) Beginning DHCP transaction (timeout in 45 seconds)

Behavior varies, sometimes the plasma widget dies, sometimes it simply does nothing (doesn't connect to the vpn, but keeps wotking). Only once it worked (it's like the 2% of the times I tried).