The "vtag" parameter of a hit refers to the version of the tag used to initiate the hit.
If the version is >5, it means that you are still using our old tracker named xtcore.js. If the version is =<5, it means that you are using our new tracker named smarttag.js
A site can be tracked with both versions of the tag if you are in a migration process. Each hit will have its version of the tag.