VaultWiki 4 should continue to be consistent with the displays of other vBulletin content e.g. not showing HTML previews where vBulletin 3 does not.
However, in VaultWiki 4, META description tags and search results snippets are first parsed in totality, then stripped of any remaining tags because they cannot contain HTML, then trimmed to the appropriate size. This takes a bit more time and processor power than simply showing the raw text, but resolves issues like showing "PAGENAME" or a raw template tag as the preview. In many cases in VaultWiki 4, this result is cached as a 'summary' field so it only has to be processed every so often.