There is not a vw_category.css template in VaultWiki since 4.0.0. If your style still includes vw_category.css, it's possible that it was customized under an earlier version or that it shipped with a custom style. In this case, you should remove this template manually, since the upgrade script can only remove default templates.
I am marking this bug as fixed in the next release, since some UI elements were still attempting to load the CSS even though it doesn't exist anymore. The references by these elements have been removed in 4.0.1.