A manpage é um pouco confusa IMHO, mas as partes principais parecem ser
--install link name path priority [--slave link name path]...
Add a group of alternatives to the system. link is the generic
name for the master link, name is the name of its symlink in the
alternatives directory, and path is the alternative being intro‐
duced for the master link.
em que generic name
é descrito na seção TERMINOLOGY
como
generic name (or alternative link)
A name, like /usr/bin/editor, which refers, via the alternatives
system, to one of a number of files of similar function.
considerando que o destino real do executável é referido como path
alternative (or alternative path)
The name of a specific file in the filesystem, which may be made
accessible via a generic name using the alternatives system.
então, no seu caso, precisa ser
update-alternatives --install /usr/bin/x-www-browser x-www-browser /usr/local/bin/vimb 30