Vtags are a capability that can be used to implement atomic updates in shared storage systems. In this case they can be used to implement atomic access to a set of shared storage devices through the storage interface. To clarify, these would be of particular use when multiple threads are using the storage interface to access local storage or when multiple servers are accessing shared storage devices such as a MySQL database or SAN storage.
This section can be skipped if you are not interested in consistency semantics. Vtags will probably not be implemented in the first cut anyway.