[Rev]

The distributor needs ported vacuum so that it advances the timing when you give it the gas from lower rpm before the mechanical timing advance advance has a chance to kick in. The manifold vacuum line that you mention goes down to the vacuum modulator on the passenger side of the automatic transmission. That controls the part throttle shifts in the transmission either upshifts or down shifts.

There is a metal line on some cars from the automatic choke that goes down to the exhaust manifold to provide heat to operate the automatic choke, but it's not a vacuum line.

Rev