I guess built-in omnicomplete is a start, but perhaps we're talking some neocomplcache plugin module or something themsuch? Googling "vim wordpress autocomplete" doesn't really give much.
Topic ide plugin-development Wordpress
Category Web
You could use the tbex.xml file from the IIS branch, and convert the CodeCompletion node to a Vim dictionary. This dictionary could be used for auto-completion.
I haven’t tested it, but if you succeed make it public, that would be awesome. :)
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