The new transfer rules do not allow transfers to take place from 31st August until January 1st when the transfer window re-opens for one month, then from 31st January to the end of the season the window closes again.
However there is a special rule which allows clubs to sign players who are un-attached when the window closes. Vidmar fits into this category after being released by Rangers at the end of last season.
The 33-year-old full-back would provide useful competition and cover for Boro's existing full-backs and would boost Boro's depleted squad, which has been been reduced to just 28 players. Of the 28, four are goalkeepers and two (Juninho and David Murphy) are long term injuries.