First off have you mapped the timing curve? I'm not a proponent of using manifold vacuum but vacuum advance at idle shouldn't make it run that bad, even if you have quite a bit of initial timing.
I have ran across a couple factory distributors that ran terrible when vacuum advance was applied and found the issue was when the vacuum can activated it closed up the magnetic pickup gap so any time I set up a factory distributor I use my mityvac to activate the vacuum advance to see if the gap changes enough to be a problem. I usually find it changes a small amount so I play with the adjustment to make sure it's within spec all the time
