You are pretty close. As I always say, engine design is a series of compromises; what works for low speed economy and torque usually hinders high speed performance and vice versa.
Large manifold volume doesn't really reduce average manifold vacuum but it does reduce mixture velocity which is just as bad at low speed. Low speed in intake plenums & runners allows gasoline to condense and settle to the bottom of the conduit messing up distribution and air fuel ratio. Also, low velocity has dynamic cylinder charging implications - cylinders aren't 'supercharged' with slow speed gas as they are with high speed gas. However, as engine speed increases, the relatively small volumes (flow area of the conduit to be more precise) in a stock type manifold begin to restrict fluid flow and hurt performance. Friction losses overpower the benefits of high velocity and cylinder filling is again less than optimal. Single plane manifolds tend to reduce low to moderate speed velocities and hurt bottom end performance relative to split manifolds but they really benefit top end performance. For big cubic inch engines that have plenty of bottom end power even with too big intake equipment benefit on the top end much more than they are hurt on the bottom end. Small engines are much more sensitive to giving up bottom end performance so more care must be taken in component selection.