A VSTi called vanguard.dll no longer opens in Cubase 6, but opens in 4 & 5. It’s reported as a missing plugin altogether in Cubase 6. I’ve used it in many songs in Cubase 6 before, but suddenly it can’t locate the plugin. It locates fine in Cubase 4 & 5.
check the blacklist. Maybe it crashed or something. Also, pop open the plugin list in Cubase and see if it is listed.