Ported vacuum should show up just after the throttle blades are opened. If it's not showing up until higher throttle openings, it could be related.

I'm assuming your timing is being ran by the distributor/vac advance, and not the Fitech unit?

As long as your idle vacuum (in D if automatic) is a few points above where the vac advance drops out, you can run it off manifold just fine.

ie, idles in gear at 13", make sure the can is 'all in' by 10ish.

You just don't want it to idle near where the can starts to move, otherwise it will really start to hunt as the can backs off, which changes the timing, which changes the idle speed, which changes the vacuum, which changes the timing