Views single field themeing with custom variables
I think it's just that there are two similarly-named theme implementations: views_view_field and views_view_fields (trailing s). Generally you can take the name of the template file before the first double-dash to find the theme implementation name, so if the template starts with views-view-field-- you'd expect the preprocess to be hook_preprocess_views_view_field(). (That naming pattern's not guaranteed though.)