Comment 100 for bug 93413

For network-manager-vpnc you can either patch the program:

--- nm-vpnc-service.c.org 2008-05-01 21:40:38.000000000 +0200
+++ nm-vpnc-service.c 2008-05-01 20:58:24.000000000 +0200
@@ -379,6 +379,8 @@ static gint nm_vpnc_start_vpnc_binary (N
  g_ptr_array_add (vpnc_argv, (gpointer) (*vpnc_binary));
  g_ptr_array_add (vpnc_argv, (gpointer) "--non-inter");
  g_ptr_array_add (vpnc_argv, (gpointer) "--no-detach");
+ g_ptr_array_add (vpnc_argv, (gpointer) "--dpd-idle");
+ g_ptr_array_add (vpnc_argv, (gpointer) "0");
  g_ptr_array_add (vpnc_argv, (gpointer) "-");
  g_ptr_array_add (vpnc_argv, NULL);

or (simpler but possibly less flexible) replace /usr/bin/vpnc with a short script which adds --dpd-idle 0 to the command line. I took the former approach, you can make up the latter...