Note: I saw this theory on TvTropes in the Danny Phantom WMG section and I wanted to write about it.
Despite their disagreements and their fights, Jack and Madeline Fenton are a happy couple. Neither of them keep secrets.
Except for one.
Maddie has loved Jack for nearly two decades. Even with his faults, there is not a cruel bone in him. He is loyal and fiercely protective of his family (despite any invention-related messes).
However, Maddie was once young. Her temper had not evened out and, just as she had done once or twice before Jazz's birth, she stormed out of their home. Yet, this time was different. She left her husband and infant daughter for a brief spell. Today, Maddie cannot honestly recall what the argument had been about. But it had been heated, and she was so devastated at how oblivious her husband could be toward her feelings - even when she knew that he truly loved her.
Throughout their marriage, Maddie has never wanted to hurt Jack, to inflict such an unforgivable blow that would crush him. And so, when she returned a roughly a week later on that distant day - fifteen years ago - her heart had sunken with guilt. Her husband had welcomed her. He was never so self-aware enough to apologize verbally, but his actions were enough. Her husband would offer her a plate full of fudge. Normally, it would amuse her.
On that particularly afternoon, it nearly caused her to burst into tears.
She never spared any details on where she went after their arguments. Jack trusted her judgment, so he never asked. That time was no exception. That would not be the last time she left the home after a disagreement or a spat, but that moment was particularly special.
"Maddie, you must be exhausted – carrying the weight of that mistake you made all those years ago."
And, flipping through a photo album of happy memories, can Maddie truly say that she regrets Danny, her only son? As Danny grows into a young man, she notices his build, his lithe frame. She notices his hands. Still, she says nothing. There's no need. They are a happy, complete family.
Inside, it kills him. Truly.
Daniel is his. His! By blood, by design. His son. It seems that even fate has given Daniel undeniable parallels to his biological father's life. How fortuitous he had thought it then - when Daniel revealed that he was a hybrid, when Vlad realized that he was no longer alone. His son's future was perfectly matched with his.
And all after that one fateful night when he ushered Madeline - his Madeline - into his home while she was shivering from the cold. (Because of that buffoon.)
And yet, as with everything, Jack Fenton has stolen what has never rightfully belonged to him. First, he stole Maddie, and now he has Daniel so blinded, so unreachable. What cruel twist of destiny would cause a rift so irreparable?
"I will have you, Daniel," Vlad mutters as he runs his hand idly down a glass capsule. The contents glow a dim green. "And if I cannot have the real you, I will settle for a perfected duplicate." Perfect, the perfect half-ghost son. Stripped of all loyalty to Jack Fenton and replaced with undying love for his true father.
"Call me 'Danny'. 'Son's' what my dad calls me."
It takes month of failures. Several upon several that he has to clean up. His failed children, dirtying his gloves with their ectoplasm. So much like blood. But no, they are mistakes. Not worthy of consideration once they destabilize. Weak. And so, with every rejection, with every setback, he works.
On an operation table inside of his cabin's lab, another Daniel dies that day. Too unstable. White hair, green eyes.
Yet, unlike the rest - the failures, the monstrosities - he reaches out, and the emotion in his eyes is so human. It instills a sort of foolish hope within a jaded man.
"Father," the dying clone says, his hand raised. When Plasmius touches the boy's cheek, the skin melts into ectoplasm beneath his palm. Just then, Vlad almost says a single name, but he refrains. This is simply another bump in the experiment. He should be furious that he has to clean up yet another mess. A stain, a reminder.
Minutes later, there is only another pile of goo where once there was a child. His child, his son.
Father.
Vlad smiles sadly, the skin of his brow creasing. "Oh, I do so love that."