It's a combination of vbox, with pack center, and align center. No flex needed.

myApp.views.defs.login  =  {
    xtype:'panel'
    ,layout: {type:'vbox', align:'center',pack:'center'}
    ,style:'background:#F0FFF0;'
    ,items:    [
            {html:'<img style="height: 24px;" src="../Resources/kblogo.png" />'}
            ,{
                xtype:'button'
                ,text:'Open Sesame!'
                ,cls:'kermitbtn'
                ,handler: myApp.views.functionality.loginSubmitHandler         
            }
            ]
}