7.1 navigation links to vertical

This is not a solution. I think Javascript is going to be the only way to get to where I think you are trying to go.

If you're going CSS only the following may get you a little closer.

@media screen and ( min-width : 641px ) {

  .header-nav-item:first-child,
  .header-nav-item:last-child
  
    {
    
    position : fixed;
    top : 35%;
    
    }
    
  .header-nav-item:first-child {
  
    left : 2vw;
    
    -webkit-transform : rotate( -90deg );
    -ms-transform : rotate( -90deg );
    transform : rotate( -90deg );
    
    }
    
  .header-nav-item:last-child {
  
    right : 2vw;
    
    -webkit-transform : rotate( 90deg );
    -ms-transform : rotate( 90deg );
    transform : rotate( 90deg );
    
    }
  }