Actually there are not more molecules to vibrate in a metal bat. Metal bats are generally hollow, and have less actual matter and mass in the same amount of space. Metal bats do vibrate more than wooden bats, but for a different reason. Metal transfers energy (both kinetic and electrical) more efficiently than wood does because metal is more uniform, condensed, aligned, and packed into neat organized packages. Wood is porous and uneven, and has imperfections. These factors are both things that help buffer against energy transfer, or in common terms, vibration. Porous materials help absorb vibrations, and imperfections like knots and grain absorb energy until the point that they fracture. Broken bats happen because the energy that would be transferred to your hands in a bat vibration got absorbed by the weak points and split the bat. Wooden bats will always vibrate less than metal bats.