[RichardJ]

The question was ask if there is manifold vacuum during cranking.
Vacuum for a carbureted engine is pressure lower than Atmospheric pressure. In order for fuel to flow from the bowl, through the jets and into the venture, the manifold pressure must be lower than Atmospheric.

Most vacuum advance units don't begin to move until vacuum reaches 6 or 7 "Hg., at which point it should only add a couple of degrees if connected to manifold , so for 200 - 250 rpm cranking vacuum should increase timing very little

Sorry Geezer, but Vacuum at WOT isn't going to tell you a damn thing. Is that WOT in neutral, in gear @ 20 mph, @ 80 mph? Useless information.

Accel and Moroso Adjustable Vacuum Advance units say to connect to manifold vacuum, but I have read that Summit suggest that you connect theirsto ported vacuum. Don't know who makes theirs.

Manifold or Ported?
Lets say you set your base timing at 10* and connected to manifold vacuum, your 20 "Hg adds 15* for a total of 25* timing at idle.
You should all agree (most will) that when you open the throttle the manifold vacuum will drop down close to 2-3 "Hg.

At that instant, the 15* vacuum advance disappears.

Mechanical advance often doesn't start until about 800 rpm(unless you have made adjustments. At 1000rpm mechanical may have added 1-2*

Lets review.
If idle is 600 it has 25* advance
At 700 it has 10 * advance
At 800 it has 10* advance
At 1000 it may have 12* advance
At 1200 it may have 14* plus maybe a couple of degrees from the vacuum advance.
Vacuum advance won't start adding in until vacuum reaches 6-7"Hg and at that only 1-2 * advance. This is a factor of where your right foot is.

Manifold vacuum turns a vacuum advance unit into a off-idle vacuum retard unit.
It becomes nothing more than a Band-Aid to stop off-idle pinging.