TL;DR: You can't really optimize the tripod apex for vibration reduction, just the legs.

Note: I've also heard the apex referred to as a spider. It's the (usually) single piece where the legs come together — the center structure.

I think you're worrying about optimizing the wrong part (i.e., the apex) for vibration. They are either cast, or milled from from a billet piece of metal. The geometry of the apex is compact and stiff. Its whole point is to be strong, in order to hold up to the stresses the legs impart on it.

A material's ability to transmit and/or dampen vibration is from deformation — the material literally elongates, bends, "squeezes", etc., tiny amounts. Absorbing vibrations is just converting kinetic energy (i.e., motion) into heat energy (albeit an imperceptible amount).

The legs of a tripod are designed to be as light as possible while spanning a long distance (relative to the cross-sectional diameter and thickness of the tubing wall). It just so happens that the choice of material in the legs also can have vibration damping characteristics, but that really just overcomes the fact that the issue of vibration was created by the long/thin geometry of the leg to begin with!

The vibrations you're hoping to reduce are actually created by the legs. As long & thin structures, they are basically oscillators "pinned" or fixed at both ends, with a certain characteristic frequency (probably below about 20 Hz). External forces, such as wind passing over the legs induce them to oscillate at their natural frequency. This oscillation is identical in nature to large structures, such as the infamous Tacoma Narrows Bridge (that literally oscillated to the point of self-destruction). Think about an oscillating suspension bridge for a moment: what can you possibly do at the nodes (i.e., the ends of the bridge) to stop the middle from oscillating? You can't. You have to address the problem along the length and/or in the middle, where the generated forces are the largest (and by reflection, where you have the most ability to impart counterforces).

Thus, it is the very material of the tripod's legs that diminishes the vibration they produce. The apex has no ability to absorb the vibrations, because it is one of the endpoints, the upper node, of each leg.