VPN actually uses "tun0" when it's connected & doesn't appear when VPN is off.
VPN can use various interfaces, because there is no single standard that defines what VPN is. You may have used a 'tun' tunnel interface, for instance, for OpenSSH, but you'd probably use gif to build one over IPsec. The manpage
$ man gif
will give you the nitty-gritty.
Such details aside, it's good to know the mystery has cleared. Thanks for letting us know.