Vertical launch needed missile control, not only deck boxes

How much earlier could vertical launch systems have become viable?

Probably not much earlier, because VLS depended on several converging technologies, especially sophisticated missile flight-control systems.

A vertical cell is only the visible part of the system. The missile must leave the ship safely, orient itself, receive guidance, and integrate with the combat system. Those capabilities depended on multiple technologies maturing together. Practical naval VLS could not move much earlier simply by imagining the launcher hardware in advance.