code: public RiakObject setVTag(String vtag)
      {
          if (vtag != null && vtag.isEmpty())
          {
              throw new IllegalArgumentException("vtag can not be zero length");
          }
          this.vtag = vtag;
          return this;
      }
docs: Set the version tag for this RiakObject
  
  @param vtag a {@code String} representing the VTag for this RiakObject
  @return a reference to this object
  @throws IllegalArgumentException if {@code vtag} is zero length