PHP has to be compiled with Multi-Byte String support, the forum has to be using UTF-8 encoding, and UTF-8 support must be enabled in AdminCP > Settings > Options > VaultWiki: Server Settings > Enable UTF-8 Support for URLs? for case-insensitivity to work properly. If any one of these is not true, VaultWiki will not be able to determine that any non-ASCII characters are the same in different cases.