While the pulse and milk tubes handle their specific jobs at the cluster level, the Vacuum Hose is the main supply line — it's what physically carries the vacuum your pump generates out to the rest of the system, including the pulsators and clusters downstream. Because it's carrying the core vacuum supply rather than a single connection point's worth, it's built heavier-duty than the smaller tubing elsewhere in the machine, and a problem with this hose has a much bigger impact than a problem with, say, one claw gasket.
If your machine's overall vacuum level seems low across every cluster rather than just one, the vacuum hose (or its connections) is one of the first places to check, alongside the pump itself. A crack, a loose fitting, or a hose that's degraded with age can all cause a vacuum shortfall that no amount of adjusting the pulsators will fix, since the problem is upstream of them.