$list=array(
      'a'=>array(
           'v'=> array('x'=>1,'y'=>9,'z'=>3),
           'f'=> array('x'=>1,'y'=>9,'z'=>3)
       ),
      'b'=>array(
            'v'=> array('x'=>1,'y'=>1,'z'=>3),
            'f'=> array('x'=>1,'y'=>11,'z'=>3)),
      'c'=>array(
            'v'=> array('x'=>1,'y'=>9,'z'=>3),
            'f'=> array('x'=>1,'y'=>9,'z'=>3)),
      'd'=>array(
            'v'=> array('x'=>1,'y'=>22,'z'=>3),
            'f'=> array('x'=>1,'y'=>2,'z'=>3)),
      'e'=>array(
            'v'=> array('x'=>1,'y'=>92,'z'=>3),
            'f'=> array('x'=>1,'y'=>9,'z'=>3)),
      'f'=>array(
            'v'=> array('x'=>1,'y'=>2,'z'=>3),
            'f'=> array('x'=>1,'y'=>21,'z'=>3)),
      'g'=>array(
            'v'=> array('x'=>1,'y'=>12,'z'=>3),
            'f'=> array('x'=>1,'y'=>1,'z'=>3)),
    );

$vy = array_map(function($x){ return $x['v']['y']; }, $list);
$fy = array_map(function($x){ return $x['f']['y']; }, $list);

array_multisort($vy, $fy, SORT_DESC, $list);