The FlexWiki plugin makes editing .wiki files much easier with Vim (Vi IMproved). The plugin provides syntax highlighting; the default textbox provided by FlexWiki has all the charm and power of Notepad. The plugin also sets a number of buffer-local options which make editing very long lines much more pleasant.
Set the HOME environment variable to point to somewhere sensible.
Create a new folder, HOME\vimfiles
Unzip flexwiki.zip in that folder.
Restart Vim.
Using the plugin
Either, edit a FlexWiki page directly, if you have access to the wiki's filestore, using :e ThatPage.wiki
Or, :new SomePage.wiki to create a new FlexWiki buffer, then copy-and-paste the text from the browser's edit window. When finished, paste the text back into the browser.
But I hate Vi!
If you don't like the modal behavior of Vim, fear not. Vim can be used as a non-modal editor using the Evim script. Evim leaves you in insert mode all the time. Or you could try Cream a much-enhanced implementation of Evim.