One common theme could be adventure. Given their backgrounds in the game world, they might be on some sort of quest together. For example, they could be exploring a new and dangerous area in search of a rare item or to defeat an emerging threat.
Another option is to check out some specific fandom forums or communities. These are often dedicated to a particular game or series that these characters are from. Members of these communities sometimes share their own fanfictions or know of great ones that might not be as easy to find on the larger fanfiction websites. You can engage with them, ask for recommendations, and who knows, you might discover some hidden gems.
One common theme is adventure. Since both characters are from the Final Fantasy world which is full of quests and battles, fanfictions often revolve around them going on an adventure together.
One common theme is friendship exploration. Since they are both characters from the Final Fantasy series, writers often delve into how their different personalities can form a unique bond of friendship.
Friendship is a very common theme. Many fanfictions explore how vincent and mike form a strong bond of friendship, supporting and trusting each other. Another common theme is growth. They often grow as individuals while also growing closer to each other in the stories.
Adventure could also be a theme. Maybe they go on a journey together in the fanfic, facing various challenges and growing closer as a result. It gives the writer a chance to develop their relationship in different settings.
One common theme is redemption. The OC often tries to help Vincent find peace and redemption from his troubled past. Another theme is love against all odds. Since Vincent has a complex and often dark history, the relationship between him and the OC is usually a struggle against various obstacles.
I'm not really sure specifically about a 'Star X Speed' story involving Jacky Vincent. There might be some niche or new concept here that I'm not familiar with.
Love against all odds is also a typical theme. There are usually obstacles in their relationship, like Vincent's secret identity and the potential danger it brings. But they overcome these challenges through their love for each other. It shows the power of love in difficult situations.
You can try fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and there's a good chance you'll find some great Vincent x Renata ones there. Another option is FanFiction.net, which also has a wide range of stories.