Do you mean should there be a vac line going to the valve cover or that your valve cover has vacuum in it, like if you pull your oil cap you can feel it sucking air in through the fill hole? You should not be pulling air in the fill hole, if you are there is something wrong.
Engine idling, disconnect breather hose at the oil fill neck, plug the the port with your finger, and vacuum should build up = no major blowby problems. The vacuum present is due to the PCV valve and it's normal. You do need to connect the breather hose to the TB...... if you leave it open, or install an open breather..... you'll have an unmetered air condition.... caused by the PCV drawing air from the atmosphere, and not from the air column already metered by the MAF.