As you may notice in Facebook's native environment, Facebook Users and Pages are able to post comments to Wall posts that include tags to a Facebook User. This causes the user to receive a notification when the comment is posted.
Unfortunately, with VSocial, at this time it is not possible to tag Facebook Users on Posts or Replies due to a limitation with Facebook's API (Application Programming Interface) that VSocial, and tools like it, interface with.
It is important to emphasize that no other 3rd party tool that interacts with Facebook is able to do this either.
We are continuously monitoring the Facebook Developer announcements for when this feature will be made available - we will be sure to prioritize it's implementation into VSocial when this happens. At this time, Facebook has not defined a date when, or if, they will make this feature available.