The Joker's secret to cheating death is...the Lazarus Pit. Tell me u didn't see that coming. If u read the latest issue of Batman, it will describe it in more detail. It makes perfect sense why Joker can't really remember his life as a family man/ failed comedian. Joker only has brief memory lapses of his past life but twists them to suit his deviant machinations.

The Lazarus Pit causes the person who bathes in it to be reborn as a blank slate in a state of madness. For Ra's al Ghul, he is snapped out of his state of madness and reeducates himself as to who he really is. The Joker however, never snapped out of his state of madness instead he continued from that night when he fell into the vat of chemicals.

The only logical explanation would be that Ace Chemicals must have somehow tapped into the Lazarus reservoir and the regenerative enzyme in the Lazarus Pit became corrupted by the sludge at Ace Chemicals. The Court of Owls must have also used the Lazarus Pit to develop the Electrum serum. The Talons can't die and they can be reanimated after being killed with just an electrical current.

Vandal Savage is a different case. The irradiated meteorite that gave Vandal his immortality must have split off into different areas around the world like Gotham. Vandal was exposed to the raw matter of the meteorite but over time, the properties of the meteorite must mineraled down into the Earth and created pools that would eventually become the Lazarus Pits.