You don't even need the temporary files:

    $ vim
    ...write, write, write...
    :w !fossil wiki create "The Foo Article" -
    :q!

Later:

    $ vim
    :r !fossil wiki export "The Foo Article" -
    ...write, write, write some more...
    :w !fossil wiki commit "The Foo Article" -

Wrapping all of that up into Vim macros would be a small matter for someone sufficiently interested.

Also, someone wanting to edit wiki-like docs in a text editor rather than in the web UI is probably going to find Fossil's embedded docs feature more useful anyway: https://fossil-scm.org/home/doc/trunk/www/embeddeddoc.wiki