Made some progress towards fixing this in the next release.
The existing META description function was not working as expected. It was using the site's
keywords tag rather than the page's summary field. For the life of VaultWiki 4 so far, the page's summary field has been a snippet of text from the first 200 characters of an article (based on historical limits of the description tag), taking it from the article's text after it has already been parsed, and then removing any remaining HTML tags. You may have seen it in search results and other places already. This was an already improved solution over the previous VaultWiki 3 behavior, despite the regression of not being able to set the description manually.
The META description now correctly uses this automatically generated summary field when it is not empty. If the summary field is empty, then it falls back to the site description.
There was previously no handling for META keywords. Now the default behavior will be to add the names of categories, tags, and other memberships (like parent area, book, or group) as keywords. If there are less than 3 keywords using this method, then it will fall back to the site keywords.
I just need to handle description and keywords using the VAR BB-Code, which I feel is the most robust solution, since it will allow modifying keywords and descriptions from even inside templates.
You can already set the title field to be different from the page's title, using
Code:
[VAR]displaytitle|Custom Title[/VAR]
This will affect the title bar, and the page's main title heading, but will not affect the URL. Note that this only has an effect when you are reading the page -- the title is not modified in any directory listings or the name that should be used to link to the page.