Comment 4 for bug 1309788

I also can confirm this with ifupdown 0.7.47.2ubuntu4.1 from trusty-updates. This vpnc configuration has been working for ages without any problems (before upgrading from saucy to trusty):

# /etc/network/interfaces
iface tun0 inet manual
    up vpnc-connect /etc/vpnc/some.conf
    down vpnc-disconnect
    up route del -host <some_IP> gw 0.0.0.0
    up route add -host <some_IP> gw 10.0.0.1 dev tun0

Doing "ifup tun0" now results in:
Cannot find device "tun0"
Failed to bring up tun0.

My current workaround is to change "up" to "pre-up" like this:

# /etc/network/interfaces
iface tun0 inet manual
    pre-up vpnc-connect /etc/vpnc/some.conf
    down vpnc-disconnect
    pre-up route del -host <some_IP> gw 0.0.0.0
    pre-up route add -host <some_IP> gw 10.0.0.1 dev tun0

ifdown seems not to be affected by this issues. "ifdown tun0" is performing vpnc-disconnect correctly.