What does the "vtag" hit parameter mean?

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.