Vulnerability is simply how much hit-points an enemy Object loses if it's hit by one of Megaman's weapons. In a vulnerability table, each byte (entry) represents an Object ID.
If the vulnerability byte is set to zero, the Object loses no HP, and the weapon Object will in some cases be thrown off in an direction towards off the screen. (like the Gyro Blade).
In some cases, the MSB bit (bit 0) in the vulnerability byte is set. This seems to indicate that a special event should incur (like the weapon is reflected, or a hit results in the enemy Object being instantly killed). More research is needed on this.