I'm having the same issue (for a while now, but I don't need VPN that much...)

Started since upgrade from 13.10 to 14.04, and still present in 14.10.

#34 Works for me (Deleting .gconf/apps/nm-applet/%gconf.xml)
#16 Doesn't work for me (Deleting keyring)

Log before removing .gconf/apps/nm-applet/%gconf.xml

--------------------------------------------
Jan  5 11:14:23 obelix NetworkManager[7538]: <info> Starting VPN service 'pptp'...
Jan  5 11:14:23 obelix NetworkManager[7538]: <info> VPN service 'pptp' started (org.freedesktop.NetworkManager.pptp), PID 8192
Jan  5 11:14:23 obelix NetworkManager[7538]: <info> VPN service 'pptp' appeared; activating connections
Jan  5 11:14:23 obelix NetworkManager[7538]: <error> [1420452863.97058] [nm-vpn-connection.c:1374] get_secrets_cb(): Failed to request VPN secrets #2: (6) No agents were available for this request.
Jan  5 11:14:23 obelix NetworkManager[7538]: <info> Policy set 'Wired connection 1' (eth0) as default for IPv4 routing and DNS.
Jan  5 11:14:27 obelix NetworkManager[7538]: <info> VPN service 'pptp' disappeared
--------------------------------------------

Logs after removing

--------------------------------------------
Jan  5 11:18:42 obelix NetworkManager[7538]: <info> Starting VPN service 'pptp'...
Jan  5 11:18:42 obelix NetworkManager[7538]: <info> VPN service 'pptp' started (org.freedesktop.NetworkManager.pptp), PID 10023
Jan  5 11:18:42 obelix NetworkManager[7538]: <info> VPN service 'pptp' appeared; activating connections
Jan  5 11:18:43 obelix NetworkManager[7538]: <info> VPN plugin state changed: starting (3)
Jan  5 11:18:43 obelix NetworkManager[7538]: <info> VPN connection 'VPN Connection 1' (Connect) reply received.
--------------------------------------------

$ dpkg -l | grep network-manager
ii  network-manager                                      0.9.8.8-0ubuntu28
ii  network-manager-gnome                                0.9.8.8-0ubuntu7
ii  network-manager-openvpn                              0.9.8.4-2ubuntu1
ii  network-manager-openvpn-gnome                        0.9.8.4-2ubuntu1
ii  network-manager-pptp                                 0.9.8.2-1ubuntu2
ii  network-manager-pptp-gnome                           0.9.8.2-1ubuntu2

$  lsb_release -a
LSB Version:    core-2.0-amd64:core-2.0-noarch:core-3.0-amd64:core-3.0-noarch:core-3.1-amd64:core-3.1-noarch:core-3.2-amd64:core-3.2-noarch:core-4.0-amd64:core-4.0-noarch:core-4.1-amd64:core-4.1-noarch:cxx-3.0-amd64:cxx-3.0-noarch:cxx-3.1-amd64:cxx-3.1-noarch:cxx-3.2-amd64:cxx-3.2-noarch:cxx-4.0-amd64:cxx-4.0-noarch:cxx-4.1-amd64:cxx-4.1-noarch:desktop-3.1-amd64:desktop-3.1-noarch:desktop-3.2-amd64:desktop-3.2-noarch:desktop-4.0-amd64:desktop-4.0-noarch:desktop-4.1-amd64:desktop-4.1-noarch:graphics-2.0-amd64:graphics-2.0-noarch:graphics-3.0-amd64:graphics-3.0-noarch:graphics-3.1-amd64:graphics-3.1-noarch:graphics-3.2-amd64:graphics-3.2-noarch:graphics-4.0-amd64:graphics-4.0-noarch:graphics-4.1-amd64:graphics-4.1-noarch:languages-3.2-amd64:languages-3.2-noarch:languages-4.0-amd64:languages-4.0-noarch:languages-4.1-amd64:languages-4.1-noarch:multimedia-3.2-amd64:multimedia-3.2-noarch:multimedia-4.0-amd64:multimedia-4.0-noarch:multimedia-4.1-amd64:multimedia-4.1-noarch:printing-3.2-amd64:printing-3.2-noarch:printing-4.0-amd64:printing-4.0-noarch:printing-4.1-amd64:printing-4.1-noarch:qt4-3.1-amd64:qt4-3.1-noarch:security-4.0-amd64:security-4.0-noarch:security-4.1-amd64:security-4.1-noarch
Distributor ID: Ubuntu
Description:    Ubuntu 14.10
Release:        14.10
Codename:       utopic