Now, this quick guide involves a lot of tools which are either experimental or are known to be buggy. Beware. Use with caution.
== NSV ==
Video is encapsulated in NSV you first need to remux it to AVI:
mencoder -oac copy -ovc copy -fafmttag 0x55 -o vp31.avi input.nsv
== Get the Theora code ==
Download the Theora source code
(svn co http://svn.xiph.org/trunk/theora ).
== Build the avi2vp3 tool ==
Copy theora/win32/experimental/transcoder/transcoder_example.c to theora/examples/encoder_example.c - this will replace the usual Theora encoder example with the application creating a Theora stream from the raw VP3 data.
Configure your copy of Theora (run autogen.sh). This has a few dependencies. The configuration process will guide you. Be patient. Once configuration didn't complain because of missing dependencies: Invoke make.
theora.ogg now should be a valid Theora + Vorbis Ogg file. Congratulations!