another stupid question

While doing routine maintenance on my 89 4.0 I noticed all the valve cover bolts were only finger tight,I put on a new gasket and tightened the bolts.
My question is,will that cause a vacuum leak?The reason I ask is I've had a whistling vacuum leak for some time,unfortunately Ican't start the motor right now to find out if the loose valve cover bolts were the source of the whistling(waiting on parts).My dad said yes it could cause a vac leak but I'm not sure because the valve cover is vented anyway(ccv)
What do you think?

I could be wrong but I think that the CCV works on pressure, so that could be the cause.

What do you mean?