There are several places. AO3, as mentioned before, is excellent. It has a wide range of stories, from the popular crossovers to more niche ones. DeviantArt can also be a source. Sometimes artists also write fanfic, and you might find some Vampire Knight crossover fanfic there. Additionally, some independent fanfic websites might have what you're looking for. However, be cautious when using these smaller sites as they may not have the same level of moderation as the bigger platforms.
Well, AO3 is definitely a top choice. It has a tagging system that makes it easy to find Vampire Knight crossover fanfic. Just search for 'Vampire Knight' and then add the name of the other fandom you want it to be crossed with. Wattpad also has some Vampire Knight crossover fanfic. You might have to dig a bit more through the search results, but it's worth a look. And don't forget about Reddit. There are some subreddits dedicated to fanfiction where you might find Vampire Knight crossover fanfic or get recommendations from other fans.
Chapter 341, ", The Parting Flute and Xiao", is the clearest early answer because it turns Yelisia's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Gu Hang's previous statement, though a sign of great respect for... and follows through with They didn't need to wait for later; their words already carried....
Chapter 241, ", My Valor Spirit Legion", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. The setup is The commander of the 31st Airborne Division, Colonel Derek, was now..., but the pressure keeps building through He was convinced that he had died before..
Chapter 224, ", When Thoughts of Attachment Arise", is the clearest early answer because it turns Primordial Wrath Owl's situation into a concrete plot event. The chapter gives readers a real answer by opening on But after the Alliance was established, their good days became... and ending on And the arrival of those who called themselves Battle Nuns in The....
Chapter 297, "The Rotten Demon Dragon", is the clearest early answer because it turns Nest Capital's situation into a concrete plot event. The chapter turns that into something concrete by moving from Although it seemed small from afar, it was in fact an enormous... to In an instant, Galaraldo already harbored a suspicion: this was very....
Chapter 295, ", Teasing, and the Million Army!", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Galaraldo could certainly see that Gu Hang now despised him to the... and ending on All you need to do is pay three billion..
Chapter 299, ", Life is Really Tough", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. The setup is Actually, Gu Hang's original plan was somewhat risky., but the pressure keeps building through Directly pulling Galaraldo's soul into the Storm God Kingdom, if....
Chapter 262, "Settlement of the Heijian Star Campaign", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. It opens with The Luman Kingdom wasn't large; in terms of size, it was about as big... and closes on However, before that, at the start of the fighting, Perbov led three....
Chapter 279, ", System Crash", is the clearest early answer because it turns Irong Fetches's situation into a concrete plot event. The chapter turns that into something concrete by moving from Although they had only taken down three bunkers for the moment, Anat... to In a certain link of logical computing, perhaps due to some....
Chapter 270, "The Demon Hunter of Blackbird City", is the clearest early answer because it pushes Bird into a new phase or setting. The chapter gives readers a real answer by opening on Ilan Bird looked up at the dark building in the rain ahead and, after... and ending on Have a cup of hot water, and then we can talk in detail." ....
Chapter 347, ", I Am the Korolya Governor", is the clearest early answer because it turns Alliance Government's situation into a concrete plot event. Early on, Meanwhile, whether Gu Hang was capable of achieving all this still.... By the end, But those could be supplemented by the Alliance's Land Forces..