I'm an experienced mechanic, but it happens to us all once in a while...
The lesson from today's work was: when you find a pair of leaking valve cover gaskets on a newly purchased truck, there is one important step to take before ordering new gaskets and taking a month to have a few hours free to clean up vacuum lines/top end while changing said gaskets....Make sure the eight valve cover bolts are more than finger tight.
Have a good day.