After Lady Vivian's third attempt to escape back to Camelot, her father locks her up in a tower with 24 hour guards. He's still convinced that this is merely a teenage infatuation, but he's becoming more concerned by the day. He even considers allowing her to go back to Camelot for half a second, but dismisses the thought, sure she'll
get over it after a few months of pining. Even though he loves his daughter, he knows as well as anyone how fickle she is.
However, this is one love that Lady Vivian is never going to 'get over'. She eats, but only because the guards force the food down her throat. Most of her days are spent at the window, staring out across the land, towards Camelot, towards Arthur. She can't sleep anymore; all she sees when she closes her eyes is Arthur's face, all she can hear is Arthur's voice in her head.
The jester said that the love potion would draw them together, but he hadn't foreseen the consequences of half the spell being broken. The Lady Vivan is slowly wasting away for want of a love that never truly existed, and there is nothing anyone can do about it.
After six months, when she is barely a shadow of her former self, her father writes to Uther that they are coming back to Camelot, and perhaps a marriage would be a good way to cement the peace. But when he goes to get his daughter, she claws at him, her skeletal frame fighting with every ounce of strength it has left. Her mind has wasted away in these last months into thoughts of Arthur, Arthur, Arthur, and now when she looks at her father, she can only see the man who tried to kill her love. Heartsick, her father lets her go, and she returns to the window, but only out of habit. She can't even conceive of Arthur in the flesh anymore, because he is so much more real in her mind. He is perfect and he loves her and she doesn't need anything else but his voice in her head and his face in her mind.
Lady Vivian dies with a smile on her face and love in her heart, and neither Arthur nor Merlin even remember what she looks like. Merlin never connects her death with the spell.