Vacuum question/ problem

ok so i have a little problem i have a place on the back of my intake manifold where there should be a vacuum line so i caped it and after i did it made the car start idling low and almost dying out so i took the cap of and it started idling like it has been again at around 800-1000rpms very confused as to why it would idle down with it on but be fine with it off.

The only repercussion with it off it whistles when i get around 5000rpms, but i did drive it a little with the vacuum cap on and it seamed a lot fast with it on but won't idle not sure why and advise would be helpful. :thumbup:

also this is on a honda civic with a b16b with stock civic type-r intake manifold.

also here is a pic of the vacuum port it is on the back left off the intake manifold near the brake reservoir.