Today, none of the (x)emacs packages in Debian know about the CSSC
directory.  Hopefully this will change.

For now, to have (x)emacs' vc mode find the commands, you should add
the /usr/lib/cssc directory into your vc-path variable.  One easy way
to do that is with:

M-x customize-variable RET vc-path RET

 -- Yann Dirson <dirson@debian.org>, Tue,  3 Sep 2013 23:08:06 +0200