The vacuum reservoir is built into the top of the valve cover. If it develops a crack it will suck in oil. You can check for the problem by pulling the vacuum hose from the vacuum pump and use a q-tip or piece of paper towel to check for oil.
If there's oil you need to replace that cover asap.
Over a cup of oil poured out if the VC when I tipped it upside down.
(If you are missing oil and not smelling it, might be where it's going).
–awr–