The Tango icon theme is missing the nm-device-wired-secure icon. This prevents the NetworkManager applet icon from indicating when a VPN is connected on a wired connection. (A padlock is normally shown over the device icon.)
Note: I haven't verified that adding an nm-device-wired-secure icon would be the right fix; I simply copied that idea from the launchpad bug report. It has been sitting there for years, so I figure some attention from upstream ought to be helpful in solving it.
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