Please add a comment for vfs.dir.size item that it is likely to be affected by Timeout settings in agent and server or proxy configuration files.

https://www.zabbix.com/documentation/3.4/manual/config/items/itemtypes/zabbix_agent

Although the same rules apply to all agent items, this one particularly if used on slow drives or large directories often might time out and the hint to configuration files can be useful.