Not sure what you're asking. At whatever junction you decide to stop the flow of vacuum (yes, I know vacuum doesn't flow, it's actually airflow the other way, but let's think of vacuum as a commodity in this discussion), it's important to cap or plug the vacuum end of the junction.

If you want to accomplish this by blocking the vacuum signal at the EGR valve, slip a bb into rubber fitting between the plastic line and the EGR valve as babymog suggests. Vacuum signal won't get to the EGR valve, air won't leak into the vacuum system. In my case, I don't have bb's lying around so I heat the tip of the plastic line with a match and pinch it sealed with my fingers. Put everything back in place for looks.

Sixto
87 300D