Fanfiction.net is also a good place. There are many different stories about them there. However, the quality may vary, so you might need to do some sorting. But it's definitely worth checking out.
One great Caitlyn and Vi fanfiction could be 'The Reconciliation'. It focuses on their relationship after a big fight in Piltover. The story shows how they overcome their differences and work together again.
A common theme is also the exploration of their backstories. How Vi grew up in the undercity and Caitlyn in Piltover, and how these different backgrounds shape their characters and interactions. Some fanfics also focus on the power dynamics between them, considering their different skills and positions in society.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are tons of Vi and Caitlyn fanfictions there. Just search for their names in the search bar, and you'll get a long list of stories. Another option is FanFiction.net, though AO3 generally has a more organized tagging system for Arcane related fanfics.
Perhaps in this fanfiction, their marriage is arranged in secret. Aegon VI might be in a precarious position, and he sees Sansa as an ally who can help him without drawing too much attention. Sansa, having learned from her past experiences with power - hungry men, is initially skeptical but realizes the potential benefits. As they keep their marriage a secret, they plot and scheme behind the scenes, slowly building their power base and making alliances. Their relationship could be full of intrigue and suspense as they try to outwit their enemies while keeping their relationship under wraps.
One possible theme could be a friendship between Brightburn and Caitlyn. They might start off as strangers and gradually form a bond. Another theme could be Caitlyn trying to understand Brightburn's powers and where he comes from.
One of the top stories could be her transition journey which has been highly publicized. It brought a lot of attention to transgender issues in the public eye.
One common theme is friendship. Many fanfics explore Caitlyn Lemon's relationships with other characters and how those friendships grow and develop. Another theme is personal growth. They often show Caitlyn facing challenges and learning from them to become a better version of herself.
Vi Warshawski is shown as a resilient and resourceful character in the novel. She doesn't back down from difficult situations and uses her skills and wits to overcome obstacles. Her personality traits and motivations drive the plot forward.
The length of the Jarilo VI story is not fixed. It could be brief if it focuses on the key events, or it could be elongated with background information and character development.