To enable MediaWiki syntax support, go to Settings > Options > VaultWiki: Legacy Options > Enable support for Legacy Wiki Markup?
Ideally, for performance, this would only be a temporary measure to make sure your pages are fully imported and will be converted properly.
To get your images and other content, you need to import the Images to your VaultWiki Image namespace, Templates to the Template namespace, etc. You'll need to create these in advance in your Namespace Manager, and make sure the "Special function" is set respectively for each one.
Also, keep in mind that vBulletin has HTML disabled by default. In many cases, HTML will be handled when the syntax is converted (VaultWiki > Wiki Maintenance Tools > Convert Legacy Syntax to BB-Code). For the rest, you should try to find BB-Code alternatives for security reasons. I would wait to run a full syntax conversion until you get all of your pages imported.