I'm not sure of a specific one off - hand. But you could start by looking on popular fanfic platforms. Check the tags related to Ventus, Vanitas, and battles. There are often top - rated or highly - reviewed fics in those areas.
Well, it could be about their early days in training. Maybe they were just little kids learning the basics of magic or combat. They could be competing in friendly ways, like who can cast the simplest spell the fastest. And they might be exploring their world together, full of wonder and innocence.
Well, it could be the dynamic between Ventus and Vanitas. Their battle is full of intense emotions and high - stakes action. The fanfic might explore their complex relationship, like the struggle between light and dark within them, which makes it really interesting.
One good one could be 'The Bond Beyond Darkness'. It really delves into the complex relationship between Ventus and Vanitas. It shows how their connection is more than just their opposing natures.
In the baby version, their relationship could be more centered around play. They might build sandcastles together and giggle when one falls down. There would be less of the underlying darkness and power struggles that are in the main story. They could simply enjoy each other's company without the heavy burdens of their destinies. For example, they could trade little trinkets they find while exploring the forest near their home as a sign of their friendship growing.
Perhaps a fanfiction where Ventus has a premonition of a great danger that only he and Vanitas can prevent together. As they journey to stop it, they gradually form an unexpected bond. They learn that their connection is deeper than they initially thought, and this realization changes their relationship throughout the story.
One way to develop their relationship uniquely is to have a magical object that links their minds. This would mean that they constantly share thoughts and feelings, whether they like it or not. At first, they fight against this connection, but as time goes on, they learn to use it to their advantage and it deepens their relationship. For example, Ventus might use it to reach out to Vanitas when he is in danger, and Vanitas might start to rely on Ventus's positive emotions to fight against his own darkness.