vacuum confusion

The other day I noticed what sounded like a vacuum leak. I opened the hood and found where its coming from. At the vacuum tree, there are a few lines that are capped off with a rubber cap. One of the capped holes has a slice in it and its sucking air in. I stuck my finger on it to close it and my car stalled. What I dont get is it it wasnt sliced open it would be closed, but when i close it now, it stalls. I just had it in for inspection last week, could there be something they did to it to cause this.
thanks

I just want to add that this just started the last few days. I have a pretty good idle too, so its not causing any problems. i just dont understand why it stalls when its closed when it always used to be closed.