Piped links should be converted to BB-Code whenever possible:
You can try running the conversion tool in your AdminCP. VaultWiki > Wiki Article Tools > Convert Legacy Code to BB-Code. (I recommend backing up your database first, in case you don't like how the conversion ends).
[[page name|displayed text]]
[wiki=page name]displayed text[/wiki]
You can also try enabling support for MediaWiki code, but this is not entirely recommended because it's slow compared to vBulletin's parser, and it doesn't cover all of MediaWiki's code. This is an option in Settings > Options > VaultWiki: Legacy Options.