I retract my previous statement thinking it was normal, and add me to the vacuum leak crew now. Here's why, hope this helps.
So Mine had done it since I got it, must have had a crack in the hard line that goes from the intake to the little junction behind the A/C accumulator, as those harder vacuum lines are often brittle and crack.
I recently had the local Chevy dealer do an intake gasket on the Astro, and they screwed up a whole boat load of stuff, but one of the things I found was my upper dash vents weren't blowing out much air at all. When I popped the hood I immediately found this open vacuum barb and a bunch of missing hose that the tech forgot to put back.
This is right behind the A/C accumulator and it actually goes from there to a little T fitting which has the super tiny hard hose going to the intake (blue line in the attached image) and also has normal rubber hose going off to the Vacuum Reservoir (red line in the attached image). All of that was missing on mine, but I bet your issue is just in the blue line being cracked.
I went to pull all this at the junkyard today and the hard line on the donor vehicle broke as well, so what I did was just use a few feet of rubber line that fit snuggle over the outside of the tiny hard line. At each end I left about an inch of hard line sticking out of the rubber 90 degree boots, then just slid the rubber line over each and routed it above my intake hose.
My A/C is blowing cold and powerful now and doesn't slow down when under a load. Hope this helps you get your issue sorted. Let me know if you need any more pictures or info.