I take it the VaultWiki 4 install was an upgrade from 4.0.0 RC 2 or something like that, that was perhaps disabled for a while? It's the only way I can see this array index possibly not being in the cache. I can fix this in the future by not bundling the cache validation into one check. It will be a little slower, but it would make sure every new field was always in there.
For now, you should be able to force it to update by disabling VW_CACHE_ON in vault/config.php, visiting one of the wiki pages with the error, then turning the cache back on.