The Vikings secondary hasn't been perfect this season by any means, but it is undoubtedly improved over last season. New coaching, a new scheme, and some new personnel will do that.
The Washington Redskins secondary wasn't very good last year either, but they haven't improved like Minnesota through the first four games of the season. Washington has the third worst pass coverage rating from Pro Football Focus this year and has suffered some key injuries in the process. Duke Ihenacho was placed on Washington's injured reserve last week, leaving them thin at the safety position.
So it makes sense that the Redskins would sign Jamarca Sanford. The former Vikings safety had an up-and-down career in Minnesota. After being drafted in the 7th round of the 2009 NFL Draft by the Vikings, he played sparingly in his first two seasons. He was prominently involved on the Vikings defense in the past three seasons, starting 41 games over the past three years. He collected two interceptions (most fans would argue that number should have been much higher if not for drops) and seven forced fumbles through the 2013 season. Sanford never really seemed to catch on with the new regime in the 2014 preseason and was placed on Injured Reserve right before the regular season started. He was then released off the IR earlier this month.
Sanford should get a good opportunity to contribute to a unit that could definitely use the help. Best of luck to Jamarca on his new team--except of course when Washington comes to Minneapolis on November 2.